Cool - mountain biking is always a cool event and would be great to go to!
With regards to camera size, I found this online:
A book out this year called Capture the Moment, a guide to taking photos at London 2012, states: "You are allowed to take camera phones, compact cameras and DSLRs into Olympic and Paralympic venues, as long as the equipment fits into a bag no bigger than 30 x 20 x 20cm. Anything above this may be confiscated and not necessarily returned to you."
Your camera bag must fit under your seat, and tripods, or big lenses, are a definite no-no as they could obscure other people's view,
Hah - and that post took me up to 300 and 'Sr Member'