I routinely reach 45, but rarely go above. At 45, the device feels warm and I kind of throttle it down a bit. However, since I've had it a long while at 600 MHz, I remember the device getting way warmer than that. So, basically, it's meant to take that kind of abuse. I doubt N900 can't take an official Nokia game for over an hour without damage.

So, basically, I keep it under 45 by feel. It is, however, true that if it reaches 45 over a longer period, it's probably that the whole board is warm, whereas an exclusive CPU task would probably mean the CPU is way above 45.

42 is fine. Sometimes it gets way hotter than that if I charge the phone after use.