Some blogs are going to call this a first. It's not, really.
Kindle hacking has been going on for 3 plus years now, and I know a couple hackers who could have pulled it off. In fact, I would bet that the second (published) Kindle hacker* could have done it. But no one ever posted the instructions on how to do it, and I never asked them.
When this idea was first possible (3 or so years ago), there was a debate behind the scenes at MobileRead, the ereader community. We decided to do the socially responsible thing and not allow the topic to be discussed. It turned out that we need not have bothered; Jesse had already reached the same conclusion.
The problem with hacking the Kindle's 3G connection like this is that the data connection costs Amazon money. As an individual, you might not care, but as a group all Kindle owners should be concerned.
If 3G starts costing Amazon too much money (because people are tethering), they might decide to simply kill off browsing over 3G entirely. And that is why I've never discussed this topic on TDR.
Folks, before you try this hack, please think about the rest of us.
Update: It looks like I might not have to worry. Hack-a-Day is reporting that Amazon can track the spike in bandwidth to individual Kindles. They might respond to just the individual hacked Kindles and not punish us all.
* The first Kindle hacker was igorsk, and I doubt that he would have told you how to do it either.