but i'm wondering... will a Celeron 2.6 or a P4 2.6 have longer battery life.. or is there another processor I don't know about (Pentium M??) that lasts even longer?
There's a Celeron vesion of the Pentium M processor that basically has half the L2 cache on it. You get much better battery life with one of them over a vanilla Pentium 4m based Celeron.
Look for a Celeron M based laptop with either 512K L2 (Banias) or 1MB L2 (Dothan).
Personally, I'd go with a regular Pentium M over either version of the Celeron.