You don't even need a printer. Shapeways - http://www.shapeways.com/ - has plenty of O scale / On30 / On3 parts available. Some reasonable, some not so much.
It has tremendous potential. In Z scale, for example, 3D printed loco designs are available that fit on a common drive. Dozens of previously unavailable engines are now within grasp. Plenty of detail parts in other scales are being designed as well.
I think 3D printing will have a huge impact on the future of our hobby.