I believe the true Mac Pro tower successor will be a "modular" system that relies on the existing iMac form factor, connected via Thunderbolt. The additional chassis will include storage bays, processing, additional I/O ports, and PCI-E lanes. The iMac will then function as a monitor with the Thunderbolt monitors available for daisy chaining. This keeps the Mac lineup firmly intact without requiring Apple to continue supporting the dedicated towers.
So in a sense, yes the Mac Pro as we know it is dead, but in another sense, the "professional Mac" configuration will be alive and well.
While you may be a little disappointed in this at first, I think all of us will grow to love the concept of a "pro box" that sits behind any iMac.