Yes, CPU speed will have an impact on battery life, but only if you use the CPU at 100% all the time, will you be able to tell the difference.
Example:
MBP with 2.26GHz has battery life of 7 hours with the CPU at 100/200% all the time, MBP with 2.53 will have maybe 6h and 40m with the CPU at 100/200% all the time.
But as you most likely only use the CPU for normal uses like browsing or writing or whatever else, the battery time will be the same, as the same processing power is used, whether it be the 2.26 or 2.53 MBP.
PS: The battery time of 7 hours is only an example, as there is no current laptop battery to hold a charge that long while being under constant full load, except if you use the MBP's battery with an under clocked Atom or A4 CPU.