Can you work with 1080p on an iMac? Depends on the codec and the drive array. The fastest array you can connect to an iMac is a FW800 one, and that pretty much limits you to compressed formats, like DVCProHD (which is 720p) or ProRes codecs. You could run uncompressed SD, but that's irrelevant here. I would pay the extra for a G-Tech drive, which I think are the best firewire and e-SATA drives on the market today. Using ProRes rather than ProRes HQ will give better performance on an iMac (as on a MBP) as it requires less decoding, and hence better real-time performance in FCP. Footage imported via log and capture or RED CINE could be played and edited on the iMac once it is encoded onto this drive.
However, without a video output device, what's the point? You can't see your footage at a proper scan rate, or size. Even if you have a 24" iMac, you can't see your footage at full size whilst you edit. So is an iMac pointless? No, absolutely not. I use mine on set as a viewing device for pre-viz and checking quality etc. That's with RED ONE footage. I need to use the Mac Pro when I actually want to work and view it at base.
If you plan on buying a Scarlet, of course you can use it to edit (as long as you have an intel iMac which supports ProRes), but unlike DV or HDV you can't use the camera as a video output device to connect to a TV for low cost monitoring. This is a problem for RED to deal with if it plans on entering the domestic market. Perhaps that firewire connection on Scarlet will support viewing video in this way. We shall see...