See, it all comes down to how lucky you are. I am much more familiar with fab processes for desktop and laptop CPUs, but I'm willing to bet money they are identical for mobile CPUs.

When AMD designs the next generation CPU, they really do not differentiate between a 2Ghz or 3Ghz part, a 1, 2, 3 or 4 core part or the amount of cache. They build the exact same CPU in exactly the same way.

Before final packaging, they test the CPU and see how fast it can go, what parts are defective, etc. Based on that they set the speed, disable some of the cores and some of the cache. This is a bit simplistic, but it generally holds true.

So if you got lucky and your N900 has a high spec CPU, then it can run forever at 1+Ghz. But you can't determine that until you fry it. However even low spec CPUs are pretty much guaranteed to run at a modest speed bump since Nokia has to leave a margin of error.

In all honesty I'd be MUCH more worried about breaking off the damn USB connector and ruining my N900 that way since it's a KNOWN design flaw.

BTW, I've been running my N900 @750Mhz and I'm sure it will survive the useful lifetime of the device. In other words everything else on it will break before the CPU fries.