I haven't really kept up with 3D printing technology, except in the most general way, so I apologize to the engineers out there,if I ask what are ignorant questions.
Most of the good plastics are two component systems. Are there two component 3D printers out there? I would imagine two component printing heads would be a nightmare to keep working reliabily. I suppose one component as a bath, and the other in the printing head would more reliable, but limited to just a few choices of materials.
Are there 3D printers capable of incorporating re-enforcing fibers or other re-enforcing materials?
3D printing as a general buzzword seems to infer a lot more capability than the technology really has today. There are also a number of different technologies employed by 3D printing. As far as fiber reinforced materials go, the only things I've seen are a fiber reinforced thermoplastic materials (run-of-the-mill ABS or similar) that is extruded out of one heated nozzle. The structural properties of these parts coming out of these type of thermoplastic machines are pretty limited. They're still good for prototyping and making cosmetic widgets but I have started to see some genuine industrial application for light duty, short run, parts. The economies are such that if you are making thousands of widgets it's still more economical to build a mold and build the parts in a more traditional fashion. However, 3D printing is slowly catching up to that model as costs come down.
Other types of very structural 3D printing are using a mixture of fine sand and fine metal powder. The sand and metal are laid down in sheets by a 3D printer and then put into a furnace to melt the metal powder into a solid unit. This is effectively making a 3D printed casting and works great for a functional item (though the cost is still too high for serial production work)