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I'm about to give up my AT&T unlimited data plan on my iPhone 4 so I can tether it to a (soon-to-get) iPad2.

I did the same, and have not regretted it.

Interestingly I find that if I tether via WiFi, my iPad doesn't always re-connect to the phone reliably after it (the iPad) has been asleep for a while. I end up having to go back into WiFi settings and connect it again. It's much more reliable in that regard if I'm tethered via Bluetooth. Speed is slower but for most of my purposes the difference does not affect my use.
 
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