It happened exactly the same with my spare N900 last week. Tried searching for a fix and i thought that was one of those weird hildon bugs but since it was my second device i've decided to reflash it. The only processes that seemed abnormal (at least that i could identify) were pulseaudio and hildon-status-menu. They both were crazy. Reboot didn't help. The only thing that i remember to install before was sshfs and the problem started right after mounting my work's box /home. Uninstalling sshfs didn't help too. Sorry. At least if i knew that i wasn't alone i'd have tried to help debuging it.