I don't know if it has less bass, but I think the Iphone 4 sounds better in general, and I'm a bit of an audiophile. I'm pretty impressed, I think it sounds fantastic.
Doing some research, it appears that the Iphone 4 (and Ipad - they share the same audio chip) really does present better audio - it's a much flatter frequency response than the 3G, which colors the audio quite a bit.
(Stolen from
here, where you can read more about their impressions.)
Compare that with the graph for the 3G:
Big difference. Not that it was bad, but the Iphone 4 is
really good.
And that might be why you think it has more bass - it has falsely bass-y audio, apparently. What you're hearing from the Iphone 4 is more natural sounding. This should be a good thing. I understand that you're used to the 3G's sound, but give the Iphone 4 some time and then listen to the 3G's output. You might understand how odd that altered sound feels to your ears after hearing something this accurate.