I found out by accident how it works. Log in to your google account, select Contacts, then set those contacts you want to export to 'Always' instead of 'Auto' or 'Never' for the option to 'Show in Quick Contacts?'.

Then go online with your N800. It should grab all those contacts if you have it otherwise set up correctly (e.g. so that the gmail notifier works, and you see your own green 'led' in the statusbar).