Thanks all for your suggestions and sympathy!

Why cant computers just do what you WANT, not what you tell them to do

I found this excellent, vegas oriented, non-techie, description...

Eugenia’s Rants and Thoughts » Blog Archive » Understanding Pixel Aspect Ratios

Basically, he (she?) says that video players are hit and miss whether they understand the Pixel Aspect Ratio. Therefore it's best to render in the actual resolution of widescreen, and not expect the player to stretch it for you.