You can't fry your CPU in the N900 even if you overclock. Well, you will probably be the first in the world maybe if it happens. This is because most modern CPUs have safety features built in. N900 will also freeze up and stop responding well before the CPU goes into some kind of melt down temperature etc. As long as you don't over-volt the CPU, it is pretty much impossible to physically damage your N900 no matter what you do in relation to frequency limit changing. The custom kernel for the N900 provides the option to under-volt but not over-volt anyways. So you are safe there. =P

Anyways, if I had to answer your question now...
No, it's not dangerous to overclock the N900.

Also, power kernel has many security measures to ensure that it is.
(e.g. It loads default value upon an unexpected "non-clean" reboot)