If you have got an iPad and now are thinking of upgrading to the new iPad 2, then the good news is that you could bring your old AT&T SIM over to the iPad 2. Because you are still using the old SIM so the data plan will still be the same.
However if you have lost the Micro-SIM or it's being damage then you could purchase it for $15. At AT&T you could get a data plan of 250MB for 14.99/month or 2GB data for 25.00/month.
If you are the type of person who don't like to pay on a month by month basis then you could opt for a prepaid option. AT&T offers a Go-Phone Prepaid Micro SIM that you could purchase for 5.49.