Well, you will find that N900 goes up to 600mhz and will stay there if it requires it. I only use mplayer and noticed it when I was playing it in the background. Also, I'm pretty sure it stays at 600mhz locked when playing game emulators. It would be too slow if it ever clocked itself down while playing.

At the end, if you manufacture a cpu to be run at a certain frequency for even few seconds / minutes at a time in a commercial device, you have to ensure that it can be run at that temperature for much much longer than what it was designed to do.

I don't know of any CPU/chip which cannot handle faster speeds when overclocked. The limiting factor is usually heat and/or voltage. I've been overclocking since the Intel 486 cpu. Every Intel/AMD cpu since then, I've overclocked at least 20% to 80% faster. My current Intel quad core CPU is overclocked from stock 2.4ghz to 3.2ghz. This is considered very "light" overclocking as this cpu can go up to 4ghz if pushed a bit harder.

I just wish I was knowledgeable enough to create an overclocker app for the N900. Unfortunately, I'm not. So I really hope that someone does.