Hey guys,
I now this Thread is very old but I just wanted to tell how you can
fix your n900 dekstop realy easy:
1. Open X-Terminal
2. make sure you're root
3. type in: kill -1 1457 <- Ends the "hildon-desktop" process
4. type in: kill -1 1458 <- Ends the "hildon-home" process
After this your desktop should reboot(not your whole device)
and then everything is like before.

I have had this problem too...almost my entire collection of widgets disappeared and I couldn't change my desktop, but now it works just like before.

Regards,
elEnemigo

P.S. Sorry for my bad english