I was down in the Dallas/Fort Worth area in 2008 for BGG (Board Game Geek convention). I tracked down the address to the id office, and got John Carmack to sign my id ball cap (had it custom embroidered). I didn't meet Carmack, but the kind receptionist took my cap back to his office, and he signed it.
https://www.blackcompany.com/images/id_cap_signed_john_carma...