I was just camping in the Beartooths and it rained ash one night. This afternoon some super thick smoke rolled into Bozeman and there was maybe a mile of visability before the sun went down. I guess I can just go DH biking with a respirator on until this clears up....
There's Big Sky for lift-accessed stuff, and there's a few other really good trails if you're willing to push your bike. I like to ride off the summit of Mt. Blackmore, personally, but the 5 mile push with a 45 pound bike is a little on the rough side.