Same trouble. Two days ago, the only alarm from calendar set for that day rings and when I pressed on "stop", it seems frozen for a while... When I get back to hildon desktop, widgets do not react any more. The only option I found is to reboot - even the "optimizer script" that restarts desktop (to cope with memory leaks) fails to refresh widgets.

My N900 is overclocked at 950 MHz and configured with swapolube. It may be a clue.