Thanks to all who replied to my mail yesterday.  My fears were 
unfounded as far as the valve went.  Someone hit the nail on the 
head with his assesment of the problem it seems.  Many thanks to all 
who replied!

Maybe VWs don't have it out for me after all...

> I'd suspect that when you swapped distributors either the timing or plug
> wires were set/put on incorrectly or the distributor nut wasn't tightened
> and it loosened up causing a backfire.  Hope it's as simple as that!

Bingo.  It turned out to be a loose distributor.  I'm not sure how, 
because I cinched it down good & tight (or as tight as I dared 
anyhow, considering the size of the clamp nut/bolt & the tendency to 
strip.  Snug.  Anyhow, a couple of miles down the road, and she 
started belching the fire of discontent.  This is info gleaned from 
the shop that looked at it yesterday.  I'm still not happy that it 
went down like this, but I'm glad to get out of the mess for $100 
instead of the $750+ I was expecting for a car worth roughly half 
that.

Thanks for the reply!

--Shaun