I ordered it on May 21st. Just came in this morning via Fed Ex. There are three options: 200MB, 2GB, and Unlimited for $15, $25, and $30 respectively. I just hope I can turn it off and on.
I've had my 3G iPad since launch and this morning and see only the 250mb and 2G plans listed. That's probably because I'm already subscribed to the unlimited plan however.