According to Firebug, here are the response headers the first time the resource is retrieved:
Cache-Control public, max-age=86400
Content-Type text/html; charset=WINDOWS-1252
Date Wed, 05 Sep 2012 15:59:31 GMT
Last-Modified Tue, 01 May 2012 05:00:00 GMT
Vary Accept-Charset, Accept-Encoding, Accept-Language, Accept
Accept-Encoding gzip, deflate
User-Agent Mozilla/5.0 (Windows NT 5.1; rv:15.0) Gecko/20100101 Firefox/15.0
Okay, I feel like a doofus but decided to put my pride aside, and rather than just deleting this question, tell what the solution was in case anyone else ever did the same thing...
Once upon a time, in order to test something, I had turned off caching in Firefox. I turned it back on, and now it is sending the header.