I was able to apply your advice to a custom child theme of the twentyten default theme that ships with WordPress 3.0, but I had to make a couple of tweaks. I’m a novice, so these may not be best practices, but I share them in case they’re useful.

First I followed the instructions here for creating a child theme.

In Step 1, I added fullwidth.php to my child theme’s folder, not to the main theme’s. (It’s still a copy of page.php from the main theme.)

In Step 3, I didn’t change style.css in the main theme, but in my child theme.

In Step 3, I copied EVERY reference to #content from the parent’s style.css to the child’s style.css. I renamed all of them from #content to #contentwide. I believe there were 62 of them.

In Step 3, instead of changing the width under #contentwide, I added the margins, from something like this in the parent style.css:
#content { margin: 0 280px 0 20px;}
to something like this in the child style.css:
#contentwide { margin: 0 20px 0 20px;}

That’s it. I haven’t tried all the features yet, but it appears to work.

Thanks again for this.