An API for general printing (e.g. business cards, letterheads, brochures, posters, etc.) is desperately needed. I can't seem to find any commercial printer that offers an API.
I haven't used the API but I've bought business cards from Moo and they were great.
I'm planning to use them to print and mail custom letters through the USPS for a credit repair site I'm building.
There are a few hurdles like dealing with large files and handling them when out of spec, but its definitely a good idea.