Re: @Craig S.

It'll hit several market niches which is something a MacPro can't do at the moment.
Smaller form factor for those who don't like towers.
High speed connectivity to a central networked brain (server san) without the local bulk.
More power and flexibility than the iMac.

[Walter Soyka] "will only increase expansion options for a single computer if you need to connect more than half a dozen high-speed peripherals."
and it may be the best solution for that market niche. It might be rack mountable in that form factor as well.

Price? The base MacBookAir at $999 has a Thunderbolt port. I don't think the port itself drives up the price that much. I haven't seen any price increase in Apple's product line as a result of the port. I think Apple believes it will increase sales. Previous computers with limited expandability now can use peripherals they were locked out of. On the other hand the new computer will have more expansion than a current MacPro and I believe will cost less for the equivalent (or better) power.

Such a box makes far more sense for Apple than a tower with PCIe slots. It'll reach users how need the power and may or may not need the expansion as well as those who want a base unit with a smaller form factor.