just some brainstorming so far:

• http://DebianArt.org/
• link to many existing t-shirt vendors on the web, to keep shipping distances short
• only link to t-shirt vendors, that provide DFSG-free designs incl. the source (as in images in sufficient high resolution at least)
• http://www.einval.com/~steve/DebianT/

• http://www.formorer.de/~formorer/DebianMerchandising/

• http://www.cafepress.com/play_machine Shirt with root password for a Debian SE Linux machine, good for trolling at conferences

Not verified list:

Debian t-shirt stores

• http://www.cafepress.com/buy/debian
• http://www.aboutdebian.com/gear.htm
• http://www.spreadshirt.net/shop.php?sid=186052
• http://www.tuxe.de/index.php?cName=debian
• http://www.elx.com.au/cat/tshirts/tlinux
• http://www.linuxemporium.co.uk/products/clothing/
• http://www.linuxpusher.dk/product_info.php?products_id=764&language=en
• http://www.spreadshirt.com/shop.php?sid=2718&search[text]=Deb
• http://www.hackerstickers.com/products/sticker-debian-linux.shtml
• http://www.thule.it/linux/listino.html#tdebian
• http://www.it2l.com/index.php?cPath=24_114 (french website)

Debian t-shirt logos

• http://www.paul.sladen.org/expo/lrlive-2005/T-shirt-logos.html