Your n900 is heating because,if you dont do the following steps, the system sets the CPU to its maximum (1150 mhz):

1) go to /system/etc/
2) search for files oc-profile-***-***.conf
3) rename one of these in "power.conf"

Personally I've renamed "oc-profile-250-500.conf" and increased by leafpad the 500 limit to 750 limit and is perfect

Note that, everytime you will use wifi in nitdroid, the n900 will still heat a bit