Troy,
Actually, I just noticed that I messed up...I meant Leopard Sharks...Tiger sharks are actually quite aggressive. At any rate it was still pretty cool...swimming around 5-6 foot sharks.
Thunderbird Mtn is pretty weak for mountain biking....but makes a great 4 mile loop to run on. I run there about once per week. You can certainly mountain bike the trail, but it really isn't much fun. Too many hike a bikes, and is generally a slow trail...plus lots of hikers/runners.
I make it out on to the trails about 5-6 times per month. The best trails I've come across are in Prescott...about an hour drive north, but well worth it. Flagstaff and Payson also have some sweet trails.
As far as local trails, my buddy and I ride the White Tanks quite a bit (far west end of Phoenix valley). They actually have about 2-3 races out there per year. It's a fairly easy trail with decent technical sections, and one big climb. It's easy to get to though.
Trail 100 is a great training ride, right in the middle of phoenix...I didn't believe it either when people first told me about it, but it really is in the middle of phoenix. Great out and back ride in the Phoenix mountain preserve. That is probably the closest good trail to campus (others may disagree). There's about 4-5 good climbs and great technical riding. Unfortunately, because the trail is in phoenix, the pollution can take a toll on ya...becomes more detrimental then good.
I almost forgot to mention the McDowell mtns, which are far east valley (East Scottsdale/Fountain Hills)...this trail system is where the Cactus Cup was ran every year (they eliminated that pro race a few years back).
Sorry to go off on the Mountain biking folks, but perhaps others besides Troy, wanted some info too.
Peace,
Shane