Originally Posted by S0urcerr0r

some of the first possible reasons i can think of:

1.) u have installed the "Recaller" and accidently hit its button on the N900 desktop... then it starts recording system sounds (or phone calls)... it can get hot if its run 2 hours straight...

4.) a webbrowser was left open with cpu intensive "flash content"...

5.) the phone lost GSM/UMTS coverage and activelly tried to trace a new cell tower...

7.) or... if its none of the above - maybe its just simply a bug in Maemo5 that for some reason hogged the system.