Simplest way... (even works between nokia and sony ericssion as my OH descovered!)

Make a backup of contacts onto your mico SD card on your old phone
On S60 goto contacts -> options->mark/unmark->mark all
then..
options->copy->to memory card.
Now..
simply pop the microSD into the N900 and restore from backup.

Dont know for sure if the N900 has the restore from SD card option but i'd be very surprised if it didnt!