one word:  RAILS

This is a new one for me.  Either I got lucky last time I built my SLI windows machine, or the cards weren't power hungry enough compared to my PSU... but in this case, it's all about the RAILS!

When I originally connected up my wires I figured given the modular design of the connectors, there was no way to wire it wrong.  Two GPUs per PSU, red goes to red.  Being a little OCD, I liked the look of VGA1 and VGA2 being populated in each PSU... FAIL!

Moral of the story... put one GPU per rail... in my case it was VGA1 & VGA3 (or VGA2 & VGA4) on the PSU.

Ugh!  No matter... I'm just glad it's solved.  Hopefully my public humiliation will help future miners avoid the same pitfall.

More reading on Rails:
http://www.hardwaresecrets.com/article/Everything-You-Need-to-Know-About-Power-Supplies/181/12