It probably does. When you port out, call them up and ask for a return kit. If they are anything like Sprint they will send you one out, you put it in the envelope (or box, or whatever they send you) and then ship it back to them (they pay all the postage).
That's exactly what I had to do when I ported out from Sprint. I requested a return kit, they sent it to me and I sent them back their Airave (femtocell/microcell).
It's only free for as long as you use it. Stop using it and they charge you. At least Sprint does anyway. But I wouldn't be surprised if AT&T is the same.
And there is really no need to keep it as it won't work with your new carrier.