The same thing happened to me this morning, plus...:
The n900 was on and charging for the night. This morning, I found it charged (green led) but was unable to unlock it. It looked halted, but I'm not sure. I powered it on and found the all my 3 desktops empty of any of my applet icons & widgets.

Moreover, the desktop menu bar (to repopulate the desktop) is completely unresponsive. I rebooted the n900, still the same.

I can no longer populate my desktops! Crazy thing.