I've got the Cthulhu Tequilla shirt. Wore it to my CoC session yesterday.
The other one I have has a little Cthulhu on the upper left chest, with "WWCD" below that, and then "What Would Cthulhu Do?" in smaller print below that. On the back, it says "Tired of some johnny come lately deity telling you what to do? Come back to the Elder ones". (I think it should say "Elder Gods" or "Old Ones", but whatever.)
Wore that to work one day and my devout catholic (although, as it turned out, severely ethically challenged) new young punk manager told me he was offended by it. "Yeah, so?"
Added: I mucked up the shirt by throwing it in after a load that had used bleach, and got some mottling on it. Almost threw it away until I got a great idea. Now whenever someone asks me what happened to the shirt, I lean in real close and with just the right look of fear in my eyes, very quietly say "I got away...".
There are simply TONS of Cthulhu t-shirts available at Cons.