I am a reasonably smart guy and know quite a bit about many things, however electronics is not one of them. Basically the extent of my knowledge as to the A2's regulation ends with. Stick little red tubes in the right way....make pretty light. /ubbthreads/images/graemlins/grin.gif
The A2 incandescent regulation is done via what is called Pulse Width Modulation. Simply put, there is a super-fast MOSFET switch which turns on and off in just the right way to maintain the equivalent of a constant DC voltage across the filament. Since the filament continues to emit light as long as it is hot, you don't notice that the current going through it is changing very rapidly (like it is in your house lights). The A2 has an LVR3L made by Willie Hunt, and it is highly efficient at transmitting every bit of power it gets from the battery to the filament. It simply meters it out carefully so that there is an effectively constant average power at the filament.
The LED's, however, are NOT regulated but are direct drive.
Also, I should mention that my M6-R is a regulated rechargeable light. and while it is TK, it is not production.
The new PT Yukon Extreme headlamp has regulation also. I checked one out at REI, but it didn't appear to be much brighter than my Vortec. Because I had to try it in the brightly lit store, I couldn't see if the spill beam is brighter than the Vortec's.