Comment 47 for bug 516412

64bit ubuntu + nvidia proprietary drivers here using nvidia gtx260oc

alt+printscrn+k restarts gdm. This restart works and no further issues until the next boot. Until plymouth is fixed later, this will bypass the lockup. The gdm restart only takes a couple seconds, so leaving plymouth installed as part of the "alpha" seems the way to go here.
No issues with gfx or other after log in is complete.