I'm a bit late on the uptake here but YES, absolutely you can mountain bike in Whistler. Are you looking for cross-country or downhill? You'll find plenty of both there.

not sure what time of year you'll be there but let's go under the assumption it's summer.

Downhill - head to the Whistler Mountain Bike Park. Not sure when it opens but you can check at www.whistlerblackcomb.com. Lots of snow on Whistler this year so the opening could be delayed.

XC - tons & tons. Go to any LBS and get some trail maps. Don't know your ability level but there's everything from easy greens, to stunts, to roots, rocks and whatever your little heart desires.

I lived there for 3 years so if you want more info, PM me.