I was contemplating this very question and believe that YES, the online T-shirt market is severely saturated.
Especially in terms of 'nerdcore' fashion in the vein of Jinx, Think Geek, and Nerdy Shirts.
I believe the key to successfully breaking into this heavily dominated market already, is with an established exclusive license on a brand or franchise that only your line will process.
Given the example of 'nerdcore' or 'geekwear', you would want to see what licenses within the related industries you can obtain to print merchandise for.
If you were interested in reaching out to the gaming niche or demographic, you may contemplate acquiring the licenses to Game Developers, popular frahcnises or consoles.
Attempt to obtain the right to print Nintendo Shirts, or Xbox 360 merchandise.
I believe with exclusive licenses to established brands within the niche you are looking to sell to, along with your own creative custom designs, is the only way to truly be successful in the heavily saturated and already dominated market of Online T-shirt sales.