I live in Canada but travel to the States. Can I just drop into a 7-11 or similar store to buy a new SIMM card to use 3G? How am I charged for use? Are the usage charges reasonable? Do I give a credit card? Is a Canadian card okay? I guess i shouldnt try using my Canadian simm card or there may be huge charges? What about countries other than the States, does it work the same in other countries?

How exactly does this all work?

Read this thread:

Traveling with the Ipad

It is a very long thread but will give you a good idea on what to expect when coming from Canada to the United States.

Basically:
1. You have to get an Ipad sims card from AT&T.
2. You have to get a Visa/Mastercard credit card with a zipcode in the US
3. Before you leave the USA cancel your 3G service.

I hope this helps. But do read through the tread as there were a lot of mistakes and frustrations in getting the Canadian iPad to work in the USA.

I signed up for Roger's Flex Rate for iPads.. They charge $20 for up to 500 MB and then $35 after that up to 5GB.

This is not a pay as you go plan.. You sign up like you would a cell phone with a credit check and all and you get billed every month. The clerk that handled my application indicated that this plan allows for roaming in the US. (But I've never confirmed it).

If it does allow roaming and depending on how bad you get dinged for roaming, it might be something to look at.. You can read about the Flex Rate here --> http://www.rogers.com/web/content/ipad-dataplans