I'll have to try this. I lost tethering when I updated my iPhone recently...

 

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When the iPhone OS 3.1 update rolled through town, it brought with it a handful of new features – but it also killed off one, not-so-official feature: unauthorized data tethering on AT&T.

 

Early this morning, the endlessly ingenious iPhone hacking community released Blacksn0w, a carrier unlock for the iPhone 3G and 3GS. Even if you have no need to plug in a different SIM card than what was originally intended, however, Blacksn0w still has its perks. Namely, it brings the aforementioned unauthorized data tethering right on back.

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http://www.mobilecrunch.com/2009/11/03/tutorial-how-to-tether-on-an-iphone-3g-or-3gs-running-os-3-1-2/