It worked!!! If you have an AT&T iPad, they are all unlocked. So you can get a T-Mobile sim card and get the free 200mb of data. Just insert the sim card and it'll ask you if you want to start a T-Mobile account. Say yes and follow the prompts to setting up the free data.
I'm soooo glad I got the LTE version now. It is so nice to be able to get my e-mails and texts on my iPad when I'm out.