To respond directly to the question posed by monfrair:
Diablo is emphatically NOT a destination for cross country riding. It has been designed for Freeride and Downhill use exclusively, and will not be your cup of tea if you are looking for the type of terrain paralleling Wawayanda, Mahlon Dickerson, KVSP etc.
Prior to the acquisition of the park by Shawn Orecchio, a small series of cross country trails existed at the summit - up until last year, Diablo offered a one-time pass up the lift for those interested in riding these trails and making their way down the mountain at the end. After exploring some of these in 2006, it is clear that this small network has been abandoned by the staff and is no longer an enjoyable ride.
Diablo Freeride Park caters to full suspension, longer travel bikes and riders looking for an adrenaline rush and to "push the envelope" of personal confidence and ability. A day pass will get you unlimited trips up the mountain via the Cabriolet gondola, and it is up to your willpower to choose the way down that best suits your riding style and experience. Whereas a day of riding cross country will tax not only your leg muscles but your cardio system, a day at Diablo is less about pedaling and more about controlling your bike, meaning your arms, shoulders and pecs suffer the most abuse.
Most frequent visitors have seen first-time or new riders at the park using hardtail or short stroke full suspension cross country bikes, along with the standard XC garb such as water bottles, XC helmets, backpacks and so forth. Though I personally got into the sport this way, and many others did as well, don't be fooled into thinking that a cross country bike is built for a lot of the rugged, steep terrain or technical drop-offs throughout the park.
The state parks in the area best cater to cross country bike riding. Don't put Diablo on your list unless you are prepared for the type of riding you will need to do to stay safe and have fun.