Turning off 3G made a big difference in battery life for me ... I spend most of my day in an office with wi-fi where my phone was constantly searching for, finding, and then losing 3G ... turning it off has worked wonders. Also my wi-fi seems more stable ... far less likely to lose my connection during the day since I switched to 2G only.
Between using wi-fi in the office and then latching on to my home wi-fi network, I'm rarely using the cell data plan. And with 2G my battery life is now 3+ days.