The UK T-shirt reviews & news bloggers at Buy-Tees.net just posted about some fantastic new printing techniques at Innovative Apparel. I particularly like their stain imaging technique, an all-over staining technique that allows a pattern to be "stained" into an entire garment.
Their stock patterns are pretty cool (see picture above, from their website). The patterns themselves can be worn as-is, or over printed with the T-shirt printing technique of your choice.
I would love to see something like this down the road for print on demand sites to offer more variety in addition to solid colored tees.