Also, the iPad's 3G data plan is different than a phone's, in that you can buy it month to month, yet never have an account either established or running. On a smart phone (in the USA) you have to have a data plan; with the iPad you don't.
So, you could not open an account. Or, you can open it for one month (for a trip or just to try out). There's people on this board who've never activated their 3G iPad (or closed the account after one month's try). The key is to turn off the account as the default is to automatically renew.
And yes, you can always turn off the 3G in the month you've activated it - just remember that the month's data allotment does not roll over to the next month.
Long story a bit longer: You don't have to have the 3G data account activated, unless you want. Then, you only have to buy it in one-month chunks.
Hope this helps.
Marilyn