What masloff outlined will only be useful (if you can call it that ) in very specific case:

This implies that you control that wifi router or have access to it. It also implies that your stolen N900 is using that wifi router - which obviously implies that it is in close proximity to the router - which leads to the conclusion you could probably SEE the N900 - or at least see it being used by somebody - and by see I mean visual eyeball contact

This implies that if you remotely reboot the N900 then when you hear the welcome theme being played you can catch the culprit who stole your phone - which implies that the N900 must be VERY close indeed for you to be able to hear it.

NOW, if you still want to know how to do it, wait for masloff's reply