There are a lot of companies doing it, check out Vistaprint for one, they will be a big competitor of yours if you get into it. A lot of them get the printing done in Europe as well, then it's shipped over.
The best and easiest way to get into the T-Shirt business is simply to start up a very quick and painless CafePress Online Shop, it only costs around $7 a month. The only thing you have to do is upload the High Res designs to the site and place them onto Products like T-shirts, sweat shirts, Beer Steins, Wall Clocks etc. even if only one person orders one t-shirt with one of your designs on, CafePress print the design on the product and send it, giving you a small % commission...
As most of you who follow my work will know, I produce T Shirts already and sell them both on my own site and my EBay shop.
Rafterman mate, having looked carefully at Cafe Press..........make sure you don't disregard going to a print shop and enquire on what kind of deals they can do for your designs. A tip would be not to go to a printer who retails designs themselves. If you want a specific process make sure they do it themselves on site not, through a third party as you're paying 2 markups. There are plenty of cheap retail 'create your own' printers who'll knock up your design quite cheaply but, you might find a few designs getting 'half inched' off you. You will need some sort of written agreement from them that your design copyright will not be breached by them.
I could go on for ages as it's more complicated than you'd think.
My artwork 'business' thus far has been a side hobby but, employment problems are making me think very seriously about how I could potentially turn it into something full time that I can make a living at....the TShirts though would only be a small part of that as it's so competitive.My designs though are unique albeit quite expensive to print currently which means I'm rethinking my strategy/policy which means knew designs. Don't ever forget the golden rule....don't sell anything you're not happy with yourself that means print quality and many test washes and ironings!!!! By all means mate, contact me on my email via my website if you have questions.