You can now start apps, they will be executed on your laptop, but all the rendering info will be sent to your N900, instead of to the laptop screen. Some apps are gltichy though, such as firefox, that somehow wants a local display, or apps like Gimp, which are not really designed to be run on 800x480.

The webcam will be piped through though, so you can start "cheese" for your own little private surveillance session

You might also want to experiment with option "C" for compression, which might improve update rate, like so:
ssh -XC root@remoteip

Of course all console commands work as usual. You could type
espeak "Nice day we are having"
to freak out whoever happens to sit at your laptop at the time.