Where to Start?
See Fowlers Attempt at putting together a printhead here. It may not be completely current but should have pictures.
There are 2 printhead designs on the internet that we appear to be able to use. The projects have recently become famous:
RepRap Probably a better printhead for developing plastic, but also more difficult to build.
Fab@Home A good general purpose printhead and extremely easy to build. I will build this first, thank you.
We would like to have a longer list of printheads. The guys at fab @home have a wonderful list here that we could work on while they try to figure out cheaper ways of making a chasis... 
In the meantime, what can we contribute?
We machinists/hobbyists can easily push the technology forward in leaps and bounds!
If you think for a minute about what is going on here, people are spending so much time on the "whole printer" that the printheads are second hand citizens.
I propose, after we get the first printheads working, that we experiment with shapes and sizes of printheads and contribute that knowledge back to the other projects!
For instance, what if instead of putting support material in the second printhead, we made that printhead put out a higher volume "plane" of material? Could we drastically shorten the fabrication time?
Think of all the cake nozzle examples available to us. What happens if we change our secondary nozzle by widening it, for instance?
We can always mill off our supports. Maybe we recommend a standard cutter/procedure for milling off plastic supports?
Can we come up with a standardization for this stuff so that we can quickly exchange printheads?
So many ideas, and so little time.