Yeah, I had read about that here:
http://www.pptfaq.com/FAQ00072.htm

But it seemed to apply only to the size of objects displayed on the slide,
not the entire page, which I'd expected to remain constant in relation to a
standard 8.5 x 11" sheet of paper.

To answer your question, though, I had tried not to open any of the files
I'm working with in versions of PowerPoint later than 2000 (for other
reasons), but I suppose it's possible that at some time I opened a PPT file
from Windows Explorer and Windows XP assumed (erroneously) that I wanted to
open with PowerPoint 2003, not PowerPoint 2000. (After which I would have
returned to open it in PowerPoint 2000, resulting in a potential conversion
twice, once in each direction?)

In any case, is there any remedy?

The thing that really baffles me is that if I open the 2 files in PowerPoint
2000, their notes page widths are different. Likewise, if I open the 2 files
in PowerPoint 2003, their notes page widths are different. At what point is
the width set and is it truly permanent? Is there any way to "trick"
PowerPoint into reinterpreting the width in one version of PowerPoint or
another?

Alternatively, if I want to make sure that the slide is centered on the page
regardless of the page width, can I get the slide width, get the page width
(minus the margins), find the difference, divide it by two and then align the
slide at that position?  If so, what unit of measure(s) do I need to use?

Any other ideas?