To save you reading through all the various threads, here is a short summary of the different mount systems available/in the works for the RED One.
Generally speaking there are 2 types of mount, electronic (aka "smart") and basic (aka "dumb") mounts. The electronic ones will let you control various aspects of motorized lenses, e.g. aperture, f-stop and possibly in some cases also stabilization technology and transfer of lens data.
Currently shipping options
RED Nikon mount
500 USD. This is a "dumb" mount, and shipping in quantities. If you order it together with your Camera, RED offers to install it free of charge. It is simple to install, takes only a few minutes. This mount does not affect your warranty. Installation video.
Alternatively, you can send your mount to Doug Underdahl, and he will add an iris control to the mount for a very reasonable fee.
OptiTek is apparently close to releasing a Pro Nikon mount, featuring a locking mechanism that gives a very tight fit for the lens.
Allstar universal A-mount
Allows using Nikon-F, Contax, M42, Leica-R and PL mounted lenses. The A-mount comes in two parts, the actual mount ($300) that goes onto the camera and utilizes the RED One's smart back-focus adjusting system, and an adapter (between $110 and $250) that screws very solid onto each lens (for the frugal souls, you can swap adapters between lenses). The lenses lock onto the mount and are a rock-solid.
Simply the most versatile, RED approved system available.
Update May 2011: The A-mount now consists of two parts, making the whole mounting system more flexible for use on current and upcoming cameras, including the RED EPIC. Announcement.
Birger Canon mount
Ca. 1.200-2.000 USD. A smart mount solution from Birger engineering, a company specializing in mounts for industrial, scientific and cine applications. Software to control it is available for the Mac and as beta for Window$. Birger is cooperating with Viewfactor, such that their Impero control knob can be utilized for lens control. The Impero can be ordered from either Viewfactor (and needs to be modified) or as a package from Birger. This combination has received rave reviews, together with some minor problems - which are being worked on. The Birger mount also transmits lens data (f-stop, focal distance, lens size) to the RED, which displays it and bakes it into the meta data. The Impero knob can be used both wireless and wired, and can be programmed in many different ways. The mount itself is as solid as the RED One is.
You have to get approved by Birger to install the mount, otherwise you will lose your warranty and upgrade path. This is a straightforward procedure.
Birger is currently shipping mounts from stock, with or without the Impero. Order it directly from Birger Engineering. Important change: As of November 15th 2009, RED no longer gives warranty to cameras with the mount installed after that date. (Link to announcement)
Birger has generated a fair amount of interest in the RED community, proof of which can be seen by the sheer size of the thread on Reduser. Full of information (and mostly historic frustration).
Birger has also come up with an adapter that will make it fairly easy to swap between Canon and PL mounts. This has been showcased in Stuttgart in November 2009. Once collaminated, swapping between Canon and PL mount will take about 5-10 minutes. It will also be possible to collaminate the PL mount with the NULL lens or the RED FOCUS.
Having a Canon mount also allows use of a variety of lenses via numerous adapters, for example Contax and Nikon mounts.
Element Technica Panavision mount
850 USD (titanum version available for 1250 USD). Not much info other than some pictures and a crucial RED approval. Available now.
VP BNCR Mount
Dumb mount. Great build quality. Full warranty.
Underdahl OCT-19 Mount
Douglas Underdahl offers the OCT-19 mount for Russian lenses for the RED One. You have to buy a OCT-19 receiver from Cinevate and then send it to Douglas. $500 later you'll get your mount. Great, friendly service. No info on warranty.
Actioncam Leica and Nikon Pro Mount
In summer 2008 Goff announced that they were working on a Leica mount for the RED One, which is now available on their webstore. Both mounts feature a lock-down mechanism, which makes for a rock-solid connection. $380.
OptiTek Nikon ProLock mount
Another mount that locks the lens. In production, cost $750. Leaves warranty in tact. Thread. Info from OptiTek.
P+S IMS mount
IMS allows you to use a variety of lenses, and also the PL mount. Shipping now. While a very good concept, the reality is that installing this mount will not only void your warranty, you will lose all future hardware upgrades, and RED will not trade in your camera for an EPIC. There are concerns about possible damage to the electronics, both by the i-cable and how the heat is transferred away from the camera's sensor, but RED Owners who have installed this mount seem happy with its performance.
Maybees and have-beens
Birger Nikon mount
Delayed. Birger is currently (October 09) working on something they call a new "iris actuator".
Unlikely to get RED approval once finished.
Alesniak Canon mount
From ca. 500 USD. Andy Lesniak offers the mount in 2 flavors, a simple dumb mount and a smart mount with an extra box and controller. The knob will allow for operation in three different modes, one for precision focus pulling, one for quick focus shifts, and a flexible mode, the latter sounding perfect for one-person-camera-departments. There will be two choices of knobs, roughly a budget and a pro version. The dumb mount will be upgradeable to a smart one once these start shipping - the smart mount is now actually available on the website, but no experiences have been posted yet.
Dumb mounts made of plastic are now shipping in numbers, and shipment of the smart mount seems to have started, though no official word has been heard from either Andy or Wicked Circuits. RED has stated that this mount is the first to be RED certified, and user installable. In most cases the installation is a relatively simple procedure that can be done by the user. Read the dedicated FAQ here.
No updates for some months...
Alesniak Nikon mount
Khmuse Canon FD Mount
Khmuse made one run of mounts for the Canon FD lens system late 2007. Voids the warranty.
Tomasson Contax Mount
DP Tómas Tómasson from Iceland once offered a bare-bone Contax mount for the RED One on eBay for $500. Will void the warranty.
Element Technica is said to be working on a mount offering, which might have something to do with their underwater housing. Continued silence.
Les Contax Mount
Les from the UK was said to be working on a Contax/Yashica mount for the RED one, but has been awfully silent the past year. Continued silence.
If you only have PL glass, the RED ONE comes with everything you need. If you only have lenses with one particular mount, the choice is relatively easy. But what if you have some stills lenses, but slowly want to move up to PL glass? If you don't have Canon glass, I'd go for the A-mount. If you have Canon, you basically have to void your warranty and go either for the Birger mount (if you want to use the electronic functions of the lenses) or the IMS mount (for quicker lens swaps).
RED Epic and Scarlet will support multiple mounts:
Of course, as with everything RED, all info is subject to change. Count on it.Sensor modules support RED, PL, Canon and Nikon mounts, which may be interchanged as required to suit a specific lens or application. Mounts include power and intelligent lens data interfaces.
If there are any obmissions/mistakes, PM me and I'll update this post accordingly. I try to keep this post updated.