Gizmodo has a good article highlighting the analysis of the iPhone 3G's battery life (some loose methodology, and some only slightly more formal) by nine industry pundit sources. All I can add to the info is that it's good to burn the batteries in for a week with full charges and discharges (even in the modern battery world) before one can really experience accurate results (batteries tend to need a couple good cycles to provide optimum output).

The general consensus? No 3G phone on the market has great battery life, but in the grand scheme of suckiness, the iPhone 3G's battery life suck the least. Forgive the terminology, please. Just trying to make a point. :)

"One takeaway seems to be that as far as straight-up 3G talk time goes, the iPhone 3G is near the top of the range—Wirelessinfo and PC World both found it to be among the best 3G handsets they've tested for voice talk time. For mixed use and browsing numbers, the range is pretty wide, since the variables at play are nearly infinite."