After purchasing several overpriced "camera" bags and using modified backpacks and messenger bags, I've finally found the answer! The Columbia Seascape. And at 20 bucks, you can use it, beat it up and throw it away when you're done - although if its like the othe Columbia stuff I have, it should last a good long time!

Holds 3 SLRs with battery grips, 4 lenses in padded cases, flashes, folded reflectors, etc - basically a mobile studio. But its not too bulky and is easy to carry fully loaded. I'm pretty psyched, because I've dealt with crappy bags for so long... just thought I'd share my find!