Had a speed wobble last night at 42mph going down a hill near Cheltenham. Probably lasted no more than 20 seconds until the bike started running straight again, but for those seconds I was cacking myself. It's only the second one I've had in 30 years' riding, and first time on this bike.
I've been looking it up on the net this morning, and the most frequently-given advice is to lift yourself slightly out of the saddle, and clamp one or both legs against the top tube to stiffen it and stop the oscillations, but these are not instinctive reactions when it happens. Maybe, just maybe, I'll be better prepared if I ever get it again.