Apple is reportedly considering a rack-mountable redesign for the Mac Pro, with one potential prototype already in circulation at the company's labs. According to 9 to 5 Mac's sources, the rackable, stackable Mac Pro is believed to be narrower and shallower than the current model - at just over 5-inches wide and 19-inches deep - with the thinking being that enterprise users could replace the discontinued Xserve with this new model.
According to the sources, the Mac Pro prototype has a new "stacked" drive system with two discs per sled, adding up to greater hard-drive density than before. The exact nature of the sleds is unknown, but Apple is believed to have designed them with both traditional HDDs and new, high-speed SSD storage in mind.
Beyond that it's all speculation, but Thunderbolt seems an obvious inclusion. 9 to 5 Mac suggests a Blu-ray drive, though whether Apple would reconsider its "bag of hurt" stance remains to be seen. As for whether this particular design will be the model to make it to production, that also is undecided; it's expected to be only one of several prototypes under consideration.