I Just tried setting mine at 1.2Ghz. N900 locked up immediately. LOL
Tried 1.1Ghz. Works. Tried playing avi. Fine. But don't feel safe. =P
So i've underclocked it (haha) back to 1Ghz which I know was stable for past 2 days.
Ah well, if I see symtoms of instability over next few days, I'll clock it down further.