it is a well accepted fact, that if you are within a relatively weak network coverage, the phone (not just the iPhone) has to "work harder" to amplify the signal.

it is a phone afterall, and doing what it was designed for, in the first place.

however there is a tip (conducted by myself and hence has no whatever scientific results to back me up) is to turn off the 3G toggle perpetually.

twittering, chatting, e-mailing in general, work pretty decently with EDGE (and even GPRS). only when i need to use safari, the 3G comes up.

you will be surprised at the power conservation with this simple maneuver. however, if you procrastinate about being troublesome et cetera, then there is nothing much that can be done i suppose...