I was under the impression you weren't able to do this on the 1st gen iphone so I assumed it would be the same with 3g. Here's a video of AT&T Chairman and CEO Randall Stephenson showed off a white 3G iPhone at NXTcomm 08 in Las Vegas where he says you can tether it.
http://www.youtube.com/watch?v=zXr0o3tR21E
http://www.youtube.com/watch?v=zXr0o3tR21E