A real beauty, presumably sold at The Dungeon Hobby Shop. The dragon illustration is by Bill Hannan, taken from the cover of Dragon #1 (1976) and later used for the cover of TSR’s Days of the Dragon Calendar (1980).
Item sold on eBay a while back for just over 20 bucks.
UPDATE (1/19/15): Thanks to some digging by Zenopus Archives at The Acaeum, Scott Moberly posted the below photo from Gen Con IX (August, 1976) showing Rob Kuntz wearing the shirt. I’ve changed the production/availability date on my post accordingly. (The Dungeon opened in April 1976, and Ernie Gygax confirmed that he remembers the shirt well.) I’ve got a Gen Con XI t-shirt here, by the way.
2 Warps to Neptune 
That’s a beauty I haven’t seen before and is now the oldest TSR t-shirt I know of. The Lizardman was the TSR Logo from mid 1975 to the end of 1978 (see this Acaeum page ). The “Come Visit My Dungeon” slogan is also on a rare sticker from the time period, distributed at the Dungeon Hobby Shop according to Ernie Gygax.
It’s beautiful. I checked the Acaeum logo page (and a few of your logo posts) before posting. I’m guessing it was sold exclusively at the shop, so it had to have been produced between April 1976 and December 1978.
Sounds right. More art trivia: The Lizardman is by Greg Bell and appeared inside the front cover of the Greyhawk supplement (1975) before being adapted as the logo. The image on the tee looks identical to the original art.
Wasn’t the Bell illustration touched up before it became the official logo?
Possibly. It’s hard to tell since the logo is so much smaller.
I found a scan I have of the April 1978 Dungeon Hobby Shop “Order Blank”, which lists a “D & D T-Shirt M, L, XL” for $4.95 – possibly this very same shirt.