RadioFlyer: As far as I'm concerned, the issue is solved.

Sheldon and everyone: I think we're looking at user error here. I have good quality Columbus forged dropouts, a good(?) Campagnolo skewer, and no adaptor claw. I have since put a new derailer on, but before I did I took it out for some serious (single-speed) mashing and it didn't budge a bit.

I don't know what was going on that day: I had the original destruction of the derailer (which I had attributed to bad derailer adjustment, but, in hindsight, may have been caused by a wheel slip) plus one or two slips while in single-speed mode. But given that it hasn't slipped since, I can only assume I wasn't clamping tightly enough.