Answer : Turn off 3G.
After the first few days of using my iphone 3G, I was rather disappointed to see that the battery would hardly last through a couple of days (with usage limited to a phone call or two, and listening to podcasts). This was a clear regression from my first iphone. Well, it seems that you can revert back to having the same battery life by turning off 3G in the settings (which is annoying but tolerable, given that I rarely access the Net from the outside, in which case I can turn it back on).
[EDIT: Firmware upgrade to 2.1 does indeed solve this problem ]