Just bought a second hand N900 and even when I switch it to idle (double press the side button), the phone stays warm and after less than an hour I find out that the battery is about to run out. Are there services/programs running whilst I have the phone off in idle mode?

What is running on your phone depends very much on your particular setup. Check what is installed, remove things you do not need, these are just basic things. However one hour seems a bit steep. Mine is also a second hand and at heavy use (e.g. downloading a large batch of maps with GPS on) the phone gets pretty hot if not ventilated properly but it takes some 5 hours for the battery to drain completely.