UPDATE [Tuesday, November 29, 2011 at 1:52pm]: Apple has yanked the app from the App Store.
This one will probably be removed any moment now, but Apple has allowed an unlimited tethering application called “iTether” into the App Store. The $14.99 application is very simple. Install the app on your iPhone, install its complementing application for the Mac or PC, plug in your USB cord and you’re good to go. Speeds will likely depend on your network connection but it’s working great for me. In fact, I’m writing this post with an internet connection through iTether. Get it while you can! (Thanks, @hrbrt)
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