I admire the Zero Punctuation shirts, particularly the "Shhh! I'm an assassin!" one. They have an inside joke on them, and a small picture if it's appropriate (I'll try and find a picture).

Something that all of us will get (No offense to D3V, but "This message is hidden because D3v is on your ignore list" is one that we'll all understand) but is still funny will be great.