There really needs to be an application that allows you to remotely locate or lock your device incase it is lost or stolen. Since I think most people will have a 3g data plan and n900 has an integrated GPS someone could program a web service that allows you to log in and track/remotely disable your precious n900. Sending the GPS coordinates using sms could also be an option. The program itself should be a console deamon or similar, so it's hidden from everyone who is just normally using the device. I think there is a similar but paid service for iPhone.

Only problem is that someone needs to develop this application and host the web service