With Kaspersky Mobile AntiVir, you can set up an "anti-theft", where you you can block the entire phone with 1 SMS & track it down with the GPS system. It's pretty sweet! But it's not compatible with the N900.

Maybe there should be some sort of software available for the N900 similar to that. (Maybe there is?)

Edit: Oh and sorry to hear that you lost it! It sucks. I've lost 2.