"In RL", there have been many Nethack-related T-shirts out there, and with advent of places where people can get any kinds of custom prints, this is even easier...
The most famous one to r.g.r.n readers, the one Marluth notes, was designed around 1998. It was designed by Sascha Wostmann and printed by Jim McLennan. In a way, it can be considered the craze starter...
The shirt had "I wished for blessed +3 GDSM and all I got was this lousy T-shirt" (with Nethack "screenshot") on front, and on the back side is "Nethack Tour 1997", with list of special levels in The Dungeons of Doom, Gehennom and Elemental Planes/Astral Plane... and Ascension in the bottom =)
Sascha has made new shirt designs for those who wish to make their own: http://www.nethack.de/shirt/shirt.shtml
...
I personally have one T-shirt, made (and advertised in rgrn in September 2000) by Harri Haataja. It is faithful to Nethack's own T-shirts: Black shirt, with white text "I explored the Dungeons of Doom, but all I got was this lousy T-shirt."