Looks like you could be in the same position as alot of Nokia 6630 owners...took a while but eventually got the new version of isync and worked like a charm, though only over Bluetooth.
I am not sure what revision of Symbian OS the N90 uses but if its the same as the 6630 or 6680 then you may be able to add it by editing the plist files of isync and sticking in the identifier of your phone. Check out Mac OS X Hints for details on how to do that, and get to the Apple discussion forums as they tend to have some peeps that are also struggling with isync and the latest Nokia phone.
Good luck and let me know if you succeed as I am seriously considering the N90 even though its meant to be rather on the large size!!!