It helps a little, but not much. As mentioned above, since you can't undervolt the idle setting (0.950), it will help only when the CPU is under load - which isn't very often during normal work patterns. I would say I'm getting maybe an extra 10-15 minutes.
Now, I bet this savings gets better for gaming and DVD watching, where the CPU is running hotter more of the time.
In any case, the bottom line is that undervolting saves you less with the core duo than it did with the pentium-M, but still helps a little.