Before it was Diablo
I used to go there every weekend with a DH specific bike (8" front and rear) and full on DH gear. My buddy would ride with me with his Specialized 3 1/2" travel FSR and pretty much keep up with me. Granted, he's a darn good rider and probably better than half the guys that call themselves freeriders, but its all about how you ride terrain. A longer travel DH bike makes up for lots of mistakes and can let you go bigger, if that's your thing, but you can do just fine on an all rigid singlespeed in you know how to ride it right. Even with my DH bike I still didn't ride every part of every trail at Diablo, and I stay away from any ledges I can't walk down without jumping (so 3 or 4 feet is my max on a good day).
Personally, I think a really good set of brakes is way more important on DH trails than suspension. This way I can stop quick enough once I realize I'm coming upon a 6 foot drop!
I've been riding my steel softail Dekerf everywhere, with my rigid fork, and the other day I passed a guy more out of shape than me at Mahlon(we were on a climb) I stopped before the rocky descent, and he tells me I've got the wrong bike for this place, I need something with 5 inches or more of travel to properly handle the trails. ( I've got a Turner Burner which I normally would ride there, but I'm still enjoying my sub-23 pound rigid (almost, anyway) bike. He started down the descent first, and I very methodically and carefully passed him with a smile. And I wasn't even going that fast. So you can ride anything, and probably still be a better rider than someone who has a bike "more proper" for a given terrain.
BTW, I'll be taking pictures of my newer DH bike and posting it in the classifieds in a couple o' days (once the rain stops). Its a 2000 Yeti DH-8, with a Rock shox Boxxer, so if anyone is interested I'll post some spam once I get the ad. Great bike for Diablo, and I'll deliver it anywhere in Jersey, or the tri-state area.