It's not zero-starting-cost custom printed T-shirts, but Berda CompuGraphix offers excellent options for orders as small as 12 units. The only decent looking printing place I could find online that would print on dark colored shirts.
This is BrainLog, a blog by Dan Sanderson. Older entries, from October 1999 through September 2010, are preserved for posterity, but are no longer maintained. See the front page and newer entries.
January 29, 2001