You definitely can if you have an iphone 4. The microsim card is common to both iPhone 4 and iPad1/2 3G. I have tried it myself - disabled wifi and inserted my iPhone 4 microsim card into the iPad 2 3G. It works.
If you ask any Apple Store staff, the standard line is misleading and nobody would give a straight yes or no.
I think we know it works, the question is can you use it and not get any notice from AT&T as others have been stating.
If you know a method that works, please let us know.