Yea mine was behaving weird, too. First of all I struggled to remove the device from isync manager. This worked however once I've reset the the sync history in preferences.
After that I've installed the new plugin. Then, it needed a few attempts to add my n900 to the sync manager. Really strange, I tried 3 or 4 times to do this before it suddenly popped up as a new device on the isync manager. Haven't done anything different than before...it was as if there was just a lag or something, no idea. Anyway, the first syncs were spitting out various error messages. This forced me to clear my ical calender and start from scratch there. This was no problem since I had all data on my n900, which i synced from.
Altogether is works now. Not 100% perfect (still struggling with events with alarm, a little bit with all day events and the calendar structure of the n900: It can only sync to 1 calendar in ical.