to OP: Although theoretically damage might occur and people here are fair enough to point out to that possibility, it seems to me like highly unlikely.

Looking back I notice I have been overclocking computer things since the year you were born and I was always happy to have extra performance to be cutting edge which is just a word for the latest outdated technology. Computers are never fast enough.
486DX50, Pentium 120, Pentium 166Mx, Celeron 300A and all successors AMD or Intel. S3 Virge videocards, 3Dfx videocards, Nvidia cards, routers, ... Never succeeded in breaking anything (except from a hard drive that couldn't keep up with the unlocked FSB)
The only thing you should care about imo is that the device continues to run stable in all circumstances to avoid data corruption.

Over here the temperature is currently 45 degrees and I carry my N900 playing a movie in while locked in my pocket on the bicycle or try to use it in bright sunlight to torture it in extreme conditions. Ideal profile 500 - 850Mhz seems reasonable speed and it is not less stable than to default config.