The t-shirt must contain a QR code, black with a (white or very clear) background ( here is one : http://mytwittshirt.com/tags/1291647970-434.png ). It must be big enough to be scanned at about one meter of distance.
Thanks for this submission! We'll care of generating the QR cod , actually we'll remove the demo one and use your design as a layer, so you don't have to worry about that; just design around the demo one.
Actually, you can slightly "hack" around the QR code to get a more beautiful result; you can alter the center of the code, or play a little bit with the colors (have a look here for some hints : http://contentdeveloper.com/2010/01…identity/). In any case, you should be able to decode the QR code either with your mobile or with an online decoding tool such as http://zxing.org/w/decode.jspx).
You must find a catchy sentence that would explain how to react when you see a facebookshirt (something better than 'scan me to add me as a friend').
Don't mention facebookshirt anywhere, it is not very relevant.
For the rest, be creative ;-), we'll be there to answer all your questions !!