OK, the only difference of note is how the plugin represents all day events. My original one used "OMA Formatting", the current one uses Midnight to Midnight. However, reading the Apple iSync SyncML Guide (http://developer.apple.com/Mac/libra...yncMLGuide.pdf), they look to be the same so the data send over the (virtual) wire is identical.

I can't offer a true solution, but if I was in that position I'd
1) Take a back up
2) Run something like this http://www.nhoj.co.uk/icaldupedeleter/ (I haven't used it myself, but it is the sort of thing to use)
3) Delete the N900 from iSync and then re-add it so you get the option to wipe it's calendar and install the iCal one as the concept of truth.
4) Cross fingers and sync.