If you lock N900 then MCE signals X11 to switch off the light and ignores any KBD press until you unlock (or slide camera cover or ...).

But processes can still consume CPU resources and application may run with full speed. For exam, Java Script is stopped in browser only after 15secs... and it seems that sometime it fails to stop ... may be fixed in PR1.2, I didn't check.

N900 lock is pretty similar to generic PC lock but it should do more in system. It is an exact reason why iPhone or Android have better standby time for less battery - if you lock them it stops Java processing completely - single task systems!