MY theory on why Apple announced the new Mac Pro today:
Apple is releasing a new Mid-tower next week and wants buyers that are hot for the pro to purchase the pro now - rather than potentially re-thing there purtchase when the mid-tower is announced.
Think about it...
But most of the Mac Pros have multi-week shipping dates, so if a new tower comes out next week, those orders can be canceled without any penalty.
And even if you did get a new Mac Pro, you can still take it back within two weeks for a refund (sans the open box fee and even that might be negotiated away depending on the retailer).
I would expect Apple to work within the existing form factors instead of introduce new ones. You could put a single socket systemboard in the current Mac Pro case designed to take a Core2 Duo or Core2 Extreme (Quad). That way, you leverage everything else about the Mac Pro, which helps control costs.