Check out this newer post  for another way of getting roaming service in the US — especially if you’re looking to use your smartphone instead of your iPad.
Every Canadian with an iPad knows this problem: regardless of which Canadian wireless provider you’re using locally, the cost of US data roaming is absurd. The best offer I’ve been able to find was something like $0.40 per megabyte for the first 100MB. After that, it goes up to $1.00 per megabyte. Lovely. I blow through that in about 6-8 hours, especially if I’m also using the device as a GPS to find my way around.
It took a bit of asking around, but the solution is as follows:
Go to any AT&T-operated store and ask for a new micro-SIM for an iPad. They hand these out for free in AT&T-operated stores (experience suggests that independent vendors feel they can charge you for the SIM, so I try to avoid those stores). After putting it into your device, you could activate a pas-as-you-go iPad data plan right on the device itself (works just like it does in Canada). The problem is, you don’t have a credit card with a US billing address, so you’re stuck.
Next, go to a grocery store or gas station, and buy a $50 Visa or Mastercard gift debit card. Make sure you don’t accidentally buy a re-loadable ‘student’ credit card (available for $4.95, often on the same wall of gift cards) because those require you to show ID when you buy them. The gift debit card will cost you the face value plus an ‘activation fee’ (mine was an additional $5.95)—I guess the retailer needs to make some money, too.
Once you get back to your hotel, you need to (briefly) get internet access; try your hotel’s ‘business centre.’ In order to use your new gift debit card for online purchases, you need to register it—which basically means you can enter any valid US address yourself as the card’s billing address. You can just use your hotel’s street address and ZIP code.
Once you’re done registering, you’re ready to use the card to activate the AT&T pay-as-you-go data plan with it. Works like a charm. Plans currently cost $30/month for 3GB of data (plenty for a 10-day trip as I just found out).
Remember to cancel your plan as you leave the US. You don’t want them to keep charging you month after month when you’re no longer there. I believe you can just re-activate it using the same method when you travel again.