Past summer I was riding down a paved descent on my hard tail mountian bike. I was going about 32 and my hands where off the bars. It freeked me out, but once I put my hands on the bars it stopped. Surprised the crap out of me.
I'll have to try and induce the shimmy again, (keeping the hands on the bar) and try Sheldon's suggestion:
Unloading the saddle (without standing up) will stop shimmy.