I'm stumped on this one...which means the answer is probably right under my nose.

I'm building a new system. Using a 790i SLI mobo with two GTX 260 graphics cards and a new Corsair "SLI-Ready" 1000w modular psu.
Each card takes two 6-pin power cables (used two 6+2 pin and two regular 6-pin). Everything is hooked up and all connections are well-seated.

Problem: The cards will not power up (fans aren't on).

Everything else powers up fine...case fans, lights, drives, etc.
At first I thought that maybe these cards just take a few moments to throttle up the fans. Nope...left it on for around 2 minutes...no fannage.

1. Tried a different power supply hooked into the mobo. Nope.
Tried switching around the 6-pin cables from the new psu. Nope.

2. Tried a different card with the new psu/mobo and used all 4 6-pin cables one at a time. It worked fine. Everything powered up. Tried all three of the PCI-e 16x slots with the different card...they all worked OK. So the problem appears not to be with the new mobo or the new psu, which led me to....

3. Tried the cards individually in a different computer. They both powered up just fine (fans came on right away BTW) and I got video on the monitor. So the cards are OK.

This mobo does NOT have an auxiliary Molex connector. The 6-pin cables provide all the power these cards need.

Frustrating......

Any ideas?