Well, not really. It's not specifically due to battery life IMO.
Faster cpu means it spends less time at high frequency.
Even konttori said it may actually not use more battery.
Also, during idle, it doesn't use a single bit of extra juice.

btw, Samsung uses the identical omap3430 at 800Mhz.
I think Nokia took the safe route by clocking it 600Mhz.
Maybe it costs them less buying them at such spec.

Either way. It's not Nokia's fault.

ALL hardware has an amount of extra tolerence that it can handle.