I think the Engadget Mobile review said something like three hours for hotspot sharing, with other light usage. That's less than you'll get on the 3G iPad, but I think it's equivalent to what you get with devices like the MiFi. Besides, how often are you using your iPad, while constantly using data, for three hours in a row, while not being near somewhere you could plug your phone in? I know it's an inconvenience, but how often would you run into it?

The only reason I'm considering eventually getting a tablet of some kind (yes, I was wrong, I want one) is because I can tether it to my phone for no additional cost. That's huge to me, and I really hope that the carriers eventually don't charge for tethering.