A good folk from wirelessinfo.com has been tested the talk time of iPhone 3G. Based on his test, the talk time is better than what Apple stated, it achieves 5 hours 50 minutes.
It’s worth noting, however, that talk time may be the least of the factors impacting your battery life on the iPhone 3G. That display seems to be a power hog, GPS is another and of course data transfer and music playback will impact it as well. Still, it’s good to see that the iPhone 3G didn’t totally tank this test given the concerns about battery life that have been voiced.