I usually use mine with my SB800 but having a sb600 too I know it fits well. The orbis has two plastic flanges each side that adjust to the size of the flashhead and I have just tried one of my sb600s and it fits fine and is secure. The orbis does take a stop or two of light so it is more the power issue with an sb600 than fit but still gets good results.
Hooray.. thanks @Paul. Yep, I can confirm that the orbis works really well with the SB600. It even comes with a small pad that you can apply if you're shooting with the smaller flashes like the SB600 and 700 to make them an even better fit with the 'flash clip'.