As we know, iPhone OS 3.0 brings with it the potential for tethering on your Apple smartphone, using the handset's 3G connection to get your laptop, netbook or other mobile device online while on the move. Apple have done all they can to make it straightforward - you can tether over USB or Bluetooth, with minimal setup - but the carriers seem less enthused. Some, like AT&T, don't officially support it yet, while others, such as O2 in the UK, will allow you to tether but have an expensive extra data plan you need to add.
One thing we know about iPhone users, though, is that if you show them a sign saying "no entry" they'll work out a hack to enable it, and sure enough there's now a simple way to turn on iPhone tethering no matter your carrier or tether-plan status. All you need do, in fact, is visit help.BenM.at on your iPhone and install a new local carrier profile configuration file.
There are files for tethering on multiple carriers - not just the official Apple partner in each company, but rivals for those using their iPhone unlocked - and more apparently being added all the time. Plus, there are install files for MMS support, including AT&T who have previously said that they won't be officially enabling it on the iPhone until late summer.
This is one of those cases where "because you can" might not mean "you should"; carriers have been very clear now and previously as to the fact that tethering data is not considered part of an "unlimited" on-device data package. If you do attempt it - and obviously there's also risk in installing any software from an unknown site - then bear in mind your carrier will likely be monitoring data traffic to identify any unofficial tethering or MMS use.