Even if looking for an extended battery is a good approach, I would suggest looking for ways to make the phone draw less power.

I'm thinking things like having a software mod that automatically switches to GSM connection when there is no data connection and a home GSM network is available, and to UMTS connection when a data connection is requested. Or severly scale back CPU when the screen is not in use . Or using a tickless kernel to reduce CPU wakeups.
I saw that there is a hot thread about powertop - we need a better tool, I think it's too focused on x86; I've been trying to use powertop on another ARM platform, and it's not that suitable to the job.
The goal would be to get 2-3 days usage with a standard battery, and then 5-6 days with an extended battery; that would start to be really portable device.