I have been overclocking all kinds of cpus and gpus for the past 15 years and have never had a chip die as a result.. Most of the overclocking have involved higher voltage as well.

The OS will always crash before anything burns.

I'll be amazed if it's different with the CPU in the N900..

I'm fairly certain that the chip is has the voltage and the frequency it has due to a compromise to make sure it's stable even if a device gets a not so good chip.

Off cause it can cause problems that'll not be noticed, like corruption, only way to check this would be a burn in test, but that could be something a bit dangerous without additional cooling I guess