Though upper arguments, some guys are still assembling their bike frame by stick welding. That´s tempting, it would allow more flexibility:
Tips for welding: http://en.openbike.org/wiki/Welding_tips
I contacted Aleksander Micsiz, from ami design (http://www.amidesign.ch/), who also stick-welded pretty nice frames - from steal squared sections - in order to have his opinion on this subject.
Here is a good reference video of a steel trike frame welded with MIG, TIG, and brazing for different types of joints.
part 1, see greenspeed.com sticker at 1:40 part 2 see TIG vs. MIG comparison @ 4:50, brazing @ after 7:30
And, for a foldable frame, used by everybody (Greenspeed, Barcroft, Bilenky,Longbikes,...), the solution seems to be the Bike Torque Coupling BTC (http://www.sandsmachine.com/)
The Longbikes model fit even in two hard cases!
And a huge idea, the Tandem convertible Trike!!!
Thank you BTC!