Seeing as how Tetrahedrons are a fairly basic building block, I'm always looking for new ways to construct them. I have been somewhat inspired by the makings of mrkim. I went out and bought some vinyl tubing to play with. while I was playing with it for motor shaft couplings, I got the bright idea that it could make for pretty easy vertices for round shafts as well.
This thing is a simple vertex constructed from vinyl tubing. There are no actual files, only pictures that show the technique.
1) Decide what size rods you're going to use
2) Purchase some vinyl tubing that is large enough to fit the rods inside snugly.
3) Cut the tubing into segments about 2" long, or whatever size you like.
4) Use an appropriate hole punch to put a roughly M4 sized hole into the middle of each of the tubes.
5) To form a vertex, just combine two tubes. Align the holes you punched, and put a screw into the hole. If you put a washer on the outside of each tube (one near the screw head, one near the nut at the end), then you'll be able to get a nice tight fit.
6) Place a nut at the end of the screw, and tighten it down until the tubes come flat against each other as much as possible.
Once you have the 'vertices', slip rods into the open tubes to form whatever space frame shape you're trying to form. For a tetrahedron, it takes equal sized rods, and only three per vertex.
Once you're happy with your construction, you can place a bit of epoxy into the tubing before you put the rods in, and that will form a nice permanent fit.
Of course this can be the basis for a PolyBot frame as it's just as stable, if not more stable, than anything you can print up in plastic.