Wow.. I think I might have these problems. ~40% of the time I reboot (which is rarely), my Mac Pro will either hang completely during the beginning at grey screen, or hang at the end, at blue screen, right before login. At first I attributed it to having my iPod plugged in (which shouldn't happen anyways), but then realized that it was only coincidence that when I took it out a couple times my baby succeeded in restarting.
I don't have any digidesign cards though- Just the stock X1900, and 6GB of RAM added. I've been meaning to wipe my HD and reinstall everything (vestiges from windows past) thinking it was some rogue program or something (VMware?), but I'm in the middle of several big projects and have to wait. When I get my copy of CS3, I'm going to do a wipe and reinstall.