Thi shas happened to me also. Im using 4.5 and once in a blue moon when I quit and reopen Maya, the undo que is turned off. I've never been able to figure it out, but then again, it doesn't happen that often so I never really worried about it. But if anyone has a solution, I would love to hear it.
Dave Baer
Professor of Digital Arts
Digital Media Arts College
Boca Raton, Florida
dbaer@dmac.edu