Based on the clock premise for life, the 125mhz less for idle will help to offset the life span- assuming operational ceiling is not compromised (> 1ghz?).

I use my N900 about three hours a day, but it is idle for the rest of the time. Since most of the day I am 125mhz less than the stock clock, this should (to some degree) offset the 800mhz factor that would not even be all of the three hours I use it.

Based on the current assumptions, I will only see about one third the life at 800mhz (if clocked there always for the three hours):

44,000 x .33 = 14,520 / 3 = 4,840 days or 13 years (even if I am off by a magnitude with the .33, it would still be years of service).

This does not consider the 50% less clock (125mhz) I save at idle, which is most of the time the phone is on.

Caveat of course is the operational ceiling of the chipset. Based on Droid, it is at some point after 1.1 ghz. Even if we assume the N900 has less efficient heat dissipation, 800mhz should be nowhere near this point.

Edit:

I did not apply the 125mhz idle time to the life, so actual life would be about 11 years- not 13 years with 125mhz-800mhz, vs 250mhz-600mhz.