If its running 2.2. I know HTC have a phone sync/switch over app. You pick your model (by Brand) and it allows you to sync data over. Not sure if N900 would be supported (as if anything supports N900!) but give that a go. Other method is to export your contacts from the N900 address book to vcf files and then import them manually from the Desire. Either using the memory card or USB connect etc.