No, I think you got the wrong guy. My shirt is unsigned.
I was one of the playtesters a few weeks back. Jon Carter gave me the shirt as a thank you present (in addition to some amazing pizza which I want to have again but forgot the name of the place you guys bought it from). I just happened to wear the shirt the day after the playtest which was the same day that I went to the XKCD Live! event at my school (WPI). I asked him what games he was excited about and Rock Band was one of the only 3 that he mentioned. That was even more impressive considering the fact that he said he's usually about 10 years behind when it comes to video game consoles. Amazing how word of Rock Band is spreading.