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Are you talking about unlimited data plan on an iphone4?

You can't set up a hotspot on the iphone4 without losing your unlimited data plan under iOS4.3. When you try to set it up from Settings->General->Network->SetUpPersonalHotspot it pops up saying you need to contact AT&T.

Either stick with 4.2.1, jailbreak, and use MiWi, or wait for an iOS4.3 jailbreak.
 
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