Dien,

Interesting idea...

As most probably know, there have been Many very Successful T-Shirt concepts like... http://www.threadless.com/news

You'll find Lots of ideas here...
http://www.t-shirt-directory.com/
http://www.t-shirt-directory.com/hot.php

I've also come across T-Shirt companies...

Making Top 10 facts like David Letterman's http://www.cbs.com/latenight/lateshow/top_ten/ on Niche Hotsheet topics...

People's Resumes & Domain name marketing ideas on T-Shirts...

Tons of Money to be made in the Marketing and Wearing of T-Shirts...

Excellent competitive marketplace...

Phil