We have a video of Steve answering this very question. Clearly I wasn’t the only one that was curious.

He doesn’t just give a simple answer why and then try and move on either, the video is actually worth a watch. The summary is that the battery life for a 3G iPhone just isn’t good enough right now.
[youtube]http://youtube.com/watch?v=0Gyq0lTubjk[/youtube]
As a bonus I put a video prior to the event at the bottom of this article, it gives you a good idea of the number of people there and of the size and appearance of the Apple Store in London. So watch and enjoy.
[youtube]http://www.youtube.com/watch?v=9fb5DThxd50[/youtube]







3g is coming in 2008 per http://www.dailytech.com/3G+iPhone+Coming+in+2008/article8933.htm
The question the article poses is “When in 2008?”