I just wanted to say a big thank you to miniME - I had similar problems / symptoms - the Calendar Process would use 80% CPU for long periods - and my N900 would become unusable - flashing didn't solve the problem either.

Eventually, with the help of this thread, I used SQLite to view the calendardb file. It had grown to 395 MB!

I copied the calendardb file to /home/user/MyDocs (took a long time). I then connected my N900 to my computer via USB - copied the file to my computer - and viewed the contents with the SQLite Database Browser (http://sqlitebrowser.sourceforge.net).

I discovered that just 3 ComponentId's in the XProperties Table took up 10,485,760 rows! I deleted all of these.

I then saved and compacted the database (calendardb) - copied it back to MyDocs on the N900 - and then copied it back to /home/user/.calendar/calendardb (after I had made a copy of the original calendardb).

The Calendar, and my N900, seem to be working much better now!

I think the problem came about because I had previously migrated recurring Calendar Tasks from my old Nokia N78.