The top-of-the-line Mac Pro has been somewhat neglected in the design department, as the current design debuted with the Power Mac G5 in June of 2003. Since then, the Mac Pro has received processor, RAM and storage updates, but the exterior design is basically the same. 9 to 5 Mac is reporting that a Mac Pro prototype is now in testing that features a narrower (just about 5 inches wide) and shorter (about 19 inches) design that could fit into a standard server rack.
This makes sense in that a rack-mountable Mac Pro could easily replace the gaping hole in the product line left by the discontinued Xserve. If the rumored design could be placed vertically for "under desk" use or horizontally rack-mounted as a server, the power of the Mac Pro could be harnessed for high-end professional work as well as for organization-wide applications.
The rumor says that the new Mac Pro features stacked drives with two drives per sled for a higher storage density than is currently available. The sleds can be configured either with SSDs or conventional hard drives. It's expected that the Mac Pro would be a 3U (5.25 inch) thick device in the rack-mounted configuration.