Promise Technologies already has
a couple options for increasing internal storage. Of course, any PCIe based storage solution will work. There are even a couple connectors with power readily available (and visible in images) but I am finding it difficult locating the page I saw previously on this. What I do not believe you will find anywhere is a sled to slide the SSD into within the Mac Pro enclosure. For that we are waiting on 3rd party solutions that will hopefully not take up a PCIe slot, though I am not sure that is possible. Time will tell, it was just released this morning.
My biggest concern is the lack of information on replacing the CPU. Even if I had to buy an Apple CPU module I would rather have that than no option whatsoever. Hopefully Apple will post definitive information on this soon, or iFixit / OWC / whatever will figure it out. In fairness I don't believe I have ever upgraded a CPU in any computer I have owned but for the new Mac Pro it would be a huge advantage being able to.