During testing for issue https://jira.reactos.org/browse/CORE-5855 I found that the behavior described in the original post has changed where Nexuiz no longer starts at all. If you issue a Ctrl+Alt+Del to attempt to recover, it results in the system becoming unresponsive.
Serge Gautherie had asked that I "move all that relates to the Mesa regression part to a new blocking issue", so here it is.
I tracked the guilty revisions to r57133 and r57134. These were built on August 22, 2012 when a lot of changes were being made to graphics code: https://github.com/reactos/reactos/commits/master?after=b5aa5bcdc1973b68dda396dc194705858d63c68d+21853
r57133 is the first time Nexuiz fails to correctly render any graphics to the screen. There are several "missing Mesa" errors before the screen freezes with graphical artifacts and the mouse cursor disappears.
r57134 is the first build that behaves like recent builds. The screen freezes, the mouse cursor disappears leaving behind a square artifact, and the VM becomes ALMOST unresponsive - the escape key will kill Nexuiz, but leave graphical artifacts like the Start menu icons being all black. Pressing Ctrl+Alt+Del makes the system completely unresponsive.
I've attached logs for Nexuiz running with the previous behavior in r57129, as well as killing Nexuiz with escape and pressing Ctrl+Alt+Del in 57133 and 57134. I also uploaded an AppCrash file from 0.4.12-dev-73-g44bb99c from other testing I did, in case it's relevant.