Hildon-desktop and xorg are running because you're looking at stuff on the display. This is typical if you have the display updating. Any time you're actively monitoring the system, just that monitoring alone is enough to cause it to use CPU.

Try running a script to see what percentage of the time it's running in idle mode. That should give you more reliable information about what your CPU is up to when the screen is off. (And no, you don't need to have the overclock kernel for the script to work, it works on any N900, you'll just have fewer states with the stock kernel.)