if you end up runnin powertop do it following way

-close all apps
-open x-term and run it and before it starts do(there is little delay before monitoring starts):
-minimize window and go to desktop view
-switch n900 to locked state (screen off) and leave it on table for couple of minutes


-open it and post results/analyze it. You should be able to see states where cpu is and what process possibly wakes processor from sleep.