Finally! A thread on this that I didn't have to start! If there is a Mac Pro redesign, Nehalem is a perfect opportunity for it.
The design itself is timeless. We've had a handled cheese grater for so long that thinking of a complete redesign is preposterous, especially when it works incredibly well for what it does. Here's my idea; it follows Apple's current trend with their machines fairly well...
Same case shape, same case style. The top handles remain handles, but are streamlined further back across the top. The bottom handles follow suit, but are solid parts of the case (It's fifty pounds, but you don't need four people to carry it). The sides are the same matte black aluminum as the aluminum iMac with a glossy black Apple logo on each side. The front and back cheese grater parts remain standard anodized aluminum. The drive doors... don't have to be matte black aluminum, but it might be a nice accent. Port-wise; two eSATA in back, another FireWire 400 and the back 800 becomes a 3200. An additional PCIe slot is added (for a total of two 16x 2.0 and three 8x 1.1).
That'd look kick-awesome, and I'd mock it up if I knew how to run Seashore well enough to not make it look like crap.
Edit: Crapped out a 5-minutes' worth mockup in Seashore. Doesn't have the side panel, so that sucks, but you get the gist of it.