@ajack: The idea is more to discourage people from being able to use your phone without reflashing (and hence loosing all your personal data). Also, reflashing isn't the most easy thing to do for a thief not to mention they aren't all that likely to find many N900s to steal. All just designed to make their life a pain.

With regards to making an app to secure your applications. It could be possible but as figaro said, constantly checking the list of running apps will eat your battery. You could have it pause checking while the phone is locked and use some tricks to drop the number of cycles performed to see what checks have to be done (say, not checking before a certain TTY which you would have checked before). I could look into doing something like this but there will always be a workaround, a simple killall command would render it useless unless it was constantly respawned (not too hard to do) but say the app doesn't check any apps which are open before it was launched, then a
killall applockerd
exec contacts && disown
has a chance of getting through (race condition). Once again though, it is unlikely that your friends are going to realise all of this, and either way, you are likely to have added xterm to your list of secure apps anyway.