There's no need to wait for Apple to release an updated version of iSync 2.1 for your Nokia N90. Navigate to Applications -> iSync -> (Control-click and Show Package Contents) -> Contents -> Plugins -> ApplePhoneConduit.syncdevice -> Contents -> Plugins -> PhoneModelsSync.phoneplugin -> Contents -> Resources
Locate Metaclasses.plist in this folder, open it in your favorite editor, and add this section of code; I used/modified the 'key' for the NOK6680.
If you notice the last string (NOK6680.tiff), this is key I used to modify the metaclasses.plist. If anyone can make/find a N90/N70 TIFF icon, please share. After making these changes, your NSeries will sync perfectly (Address Book and iCal). Tried and tested on my N90 with firmware version V.2.0530.3.5, dated July 29 2005.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051007035617401