Gotta have something to reference Austin Powers Goldmember line of:

Preparation H: It does feel good...on the hole.

Ok here's an oldie

Papa Smurf saying "There are 100 [or some other number] Male Smurfs and 1 Female Smurf. We're blue for a reason."

How about a lost shirt? I have an idea for a couple and one goes somehting like this

Lost fan- Someone that understands what's happening without repeating an episode; someone that gets annoyed with another person that doesn't understand until season six.

like battlefield 2 as a soldier waiting in a waiting room with alot of others and on a sign saying 10 seconds or so to respawn

woot about like a mmorpg like wow standihe hordes needs you, with like a uncle sam looking orc pointing at your direction

lets say the hidden blade from assassins creed pointing upwoard saying like *!@$ you

Nightcrawler bamfing above a soldier/marine/master chief/whatever and dropping a grenade. On the bottom it reads "Telefragged!"

A conversation between PC and Mac.
Mac: Oh hey PC, what are you doing?
PC: Playing a game.
Mac: Which game.
PC: All of them.

PC's. Because we can right click.

How about a picture of a Big Daddy and on the bottom it reads "Who's your Daddy?"

How about a shirt that reads "To Gaming!!" and it shows a picture of 3 hands, each holding a different game controller. One has a Wiimote, another has the 360 controller, and the other one has the PS3 controller. And if you want, I guess you can have someone holding a mouse (You know, for PC gamers).

How about a shirt that has a Pokemon (It could be any Pokemon that evolves) evolving and on the bottom it reads "Evolution does exist." I know it might be a little offensive but it's kinda funny also.

How about a shirt that says "Never forget your roots" and it shows a picture of a plant that has a 360, Wii, and PS3 as leaves and the roots are systems like the Odyssey, Atari 2600, and other old consoles. That would be kinda cool.

Mass Defect (in style of Mass Effect naturally)
Something to tie in the release of Gamma Radiation from a nuclear reaction

A t-shirt with Master Chief on it, being awesome. Because we don't have one, and I just want to freaking buy a shirt with MC on it.

That Ico T-shirt is great is it possible to get a poster or mousepad?

retro video game characters zombified? watch out for your brains...

plz leave suggestions for improvements...

An idea below made me think of this, the Juggernaut symbol from Modern Warfare with "I'M THE JUGGERNAUT, BITCH!" underneath it.
Anybody who doesn't get it, watch this video:
http://www.youtube.com/watch?v=bSuvOVH0aSQ

Mass Effect shirt that has a larger weighted person with planets and the normandy floating around him that says "Mass Effect"

How about a sausage, a pickle, and a knife? Get it?

we need muffins. Or rule #34, lol.

Aberzombie and Flesh* Not Fitch...totally messed up original idea post...

women get underwear, why not make mens boxers, that would be great

my idea is ties, for gamers that work in an office environment. and also for someone like me, because every wendsday for school we must dress in a suit. so i think gamer ties would make a good buy for a lot of people.

Front: a Split Reason logo on the breast
Back: "Sleep? Psh" (2 lines)

what about a shirt that says "i sold my life for my xbox" and on the back "and it was worth it"