Gizmodo compiled this impressive chart of battery times for the iPhone 3G as reported by various reviewers.
Most reviewers found the iPhone 3G did have around 5 hours of talk time which was amongst the best for 3G handsets that were testing. The "mixed" use included web browsing and had far more variable results. In those tests, the iPhone 3G ranged from 2 hours and 52 minutes of use as high as 5 hours and 49 minutes.
Hardware site Anandtech actually performed dedicated web browsing tests for the iPhone 3G compared to the original iPhone on EDGE. They found that iPhone 3G (in 3G mode) could browse for 3 hours and 17 minutes, while the original iPhone browsing on 2G could browse for 5 hours and 42 minutes.