Penryn will be even more optimized for mobile computing and as the chip has been designed for mobile computing it will be even better on your battery than Merom, the current Core 2 Duo. First of all we are talking about 45 nanometre part and David Perlmuter aka Dadi the Exectuive VP or Intel's mobile group said that Penryn will be able to work wit even lower voltages and that the clock will be able to drop even higher.
If the machine gets to idle mode Intel will try to shut down most of the cache and therefore save as much power as possible. The CPU will drop the clock even deeper and we can expect some few hundred MHz clocks if the machine is not doing some tough job.
The CPU needs to empty the cache first before shutting it down, and we are sure that this can't be that good for the performance. I guess that Intel has found the way quickly increase the clock speed and fill the cache if necessary but according to a few engineers we talked to there wont be much better battery life between Penryn and Merom based machines.
This is what the new power state looks
Cache on when plugged
Cache off to save the battery