There is nothing in the kernel to reduce frequency from 600MHz under load. I bet there is also none in user space, as it does not make sense to put it in user space. I ran simple program to use 100% cpu time for 3 hours, and all 3 hours were at 600MHz. If your metalayer-crawler (or any other program) goes crazy, and you put the N900 to charge and go to sleep, then it spends all the night in 600MHz (and 1.375V), and generates a lot of heat. It must have happened to thousands of people, yet we do not here about so many dead N900s, therefore the N900 must be a bit less sensitive to heat than you imagine.

System heat is a bad argument against overclocking. There are a few worse heat generators in the N900, including the battery and the UMTS radio.

In addition - for a given task the heat generation is almost independent of frequency, since power used is proportional to frequency, but time needed is inversely proportional. Heat increases proportionally to square of voltage, and most overclockers use lower voltage then default Nokia settings, thus using less power, and generating less heat.

Of course, some activities (such as some games or video playing) might use the additional frequency to do more, and thus generate more heat for apparently the same work. But for more normal tasks (such as web browsing), overclocking results in less heat generated and less battery used, as almost any overclocker reports.

Droid, with the same CPU, designed by Motorola for running at a maximum of 550MHz is already being overclocked for over 3 months, without any reported ill effects.