This was briefly mentioned in another thread, but there was a recent news story on it, so I figured it deserved its own thread.
For those people that have the t-shirts printed and they have their own website and merchant account to process transactions -- but don't want to warehouse and ship t-shirts, this new amazon.com service might work for you:
Amazon Introduces Fulfillment Web Service -- E-Commerce -- InformationWeek Amazon.com: Amazon Web Services - new: Help: New FAQ & More