There is a learning curve to screenprinting, and process printing is basically at the top of that curve (i.e. it's the hardest thing to learn). So yes, full colour is hard.I like the idea of screen printing but I'm not sure I have the space or budget to buy the equipment... and wouldn't it be hard to do full colour designs?
Outsourcing the printing is perfectly cost effective if you're printing in enough quantity though.
Yes, Threadless (and almost all companies like them) use screenprinting.What do companies like Threadless do? I presume their designs are all screen printed?