Have you read the thousands of messages in the overclocking thread?
I don't know enough about the gritty details, but on the Summit, at the OMAP3 power presentation Igor Stoppa (and he's no n00b when it comes to Maemo HW ) very explicitly said - please don't overclock it - it WILL fry.
Overclocking is not in fact ill-advised.
If it is, you should immediately advise Nokia, because it is my understanding that the N900 has ALREADY been overclocked to a certain extent by Nokia.