As soon as I get my n900 - if there will not yet be such a software - I will definitely write an app to achieve just this. It will be a daemon running in the background checking for a single character in the web domain somewhere. if your n900 gets stolen, you can set it to something else then 0 to activate the tracking. It will send GPS info over the internet and SMS as well as pictures taken using the front camera - possibly to grab a picture of the culprit. You will also be able to instruct it to upload specific data at this point at least to save important data if you don't recover the phone, and gain ssh access.

At least this is what I'm planning. I can only start to go into details when I finally get the phone.