Thanks Corion, your suggestion pointed me in the right direction.

I compared the two response headers (my test and Google's) and there were additional headers being sent by the web server I was using (that were not in the script).

Apache's config file had several "header set" directives added to prevent cacheing (that I was unaware of). I'm glad the problem is solved, but I was barking up the wrong tree! If I hadn't looked at the headers the way you suggested... well, I'd probably still be pulling my hair out!

At any rate, thanks for the help! Cheers!