Frozen menu bar
I keep getting this problem when coming out of sleep on my MBP running Snow Leopard. What happens is that the menu bar stays locked on an application (say Firefox for example) and no window (not even Firefox) can gain focus. The only way to fix this is to hold down the power button and reboot.
If you force quit (command, option, escape), does it show which app is not responding? That might point you in the right direction of where the problem is.
apparently nothing has stopped responding?!
You could try relaunching Finder, that may unlock the menu bar, although not solving your problem entirely.