OK, I've got something working with Snow Leopard and PR1.1. You can find it at http://www.smithfamily.org.uk/~dgs/N900iSync/

Please heed the warnings, back up EVERYTHING before you even think of installing this. As it says in the notes, events created on the N900 don't appear to get pushed back to the Mac. I've also included the iSync Plugin Maker source so if you want to, you can have a go at making it better.