SwagDog.com -- apparently they've teamed up with Blizzard to offer real-life versions of your guild tabard on a t-shirt, and folks who attend BlizzCon will get their pick of the first shirts off the presses.
Note to Blizzard and SwagDog: feel free to send the royalty checks over to me whenever you want. The bad news is that they've eschewed the idea of "t-shirt as tabard", and instead it looks like they're making a custom-printed shirt for everyone, so it'll have not just your tabard on their shirt, but your guild's name, and your character's name and realm, race, class and faction as well.
As neat as these are, I think I'd like something a little more subtle -- all that text and the icons muddy it up a bit. But otherwise, definitely seems like (assuming the prices and shirt quality are respectable) a fun way to show off a little guild and toon pride.