Using a 3G phone as modem for a notebook when needed is something I totally assume to be possible as I do it all the time here in Germany. Now a report that AT&T is not allowing future iPhone 3G owners to use the iPhone 3G to get their notebooks connected according to a widely circulated story on iPhoneAtlas.

AT&T is offering special modem (PAM) plans for BlackBerry and Windows smartphones. It is not clear yet what Apple and AT&T would do to prevent iPhone 3G owners from using it as a modem via Bluetooth for instance.
9to5Mac reports in the meantime that O2 in the UK will totally allow iPhone 3G tethering over Bluetooth. I do not think that Apple will release a special iPhone 3G version for the UK that allows this, so only AT&T could do something on the network side to prevent tethering.
The original iPhone had to be hacked to allow tethering, but without 3G that was not that interesting.
More iPhone 3G News.