What is it about 3G that kills the iPhone battery? Why does turning on 3G data make my iPhone's battery die faster?
The 3G radio requires more power. If you live in a weak 3G signal area, it may also cycle a lot between EDGE and 3G; setting up the 3G connection draws more power. Finally, you may be able to save power by setting the iPhone to Fetch New Data less often (Settings > Mail, Contacts, Calendar > Fetch New Data).