And yet another type of "geeky" hat (in line with the pi hats and others)

Morse Code Hat on craftster.

The message spells out "I am not wearing underwear today."

(I probably would have come up with something much geekier: like "If you can read this, you must know Morse Code." or maybe go all semi-Magritte-ish and have it say "This is not a hat")