Nothing lasts forever , I highly doubt people will use this phone more then 3-4 years besides Cortex A8 in different variations ( Apple A4, Samsung Hummingbird ) etc are fully capable of running at 1Ghz so N900 with its 600Mhz can be easily overclocked to 850 Mhz with even somewhat reduced voltages (lv mode in Titan's kernel). Unless you increase voltage insanely high there should not be any hardware problems , not sure about software because it was discussed in other thread if clock is too high some process might give errors skip cycles etc

In summary ,
Overclock and enjoy if you fry yours please post here