Years ago, Googlebot would send If-Modified-Since headers, and Apache would honor them.
I ran into this by chance when writing a wrapper to obfuscate e-mail addresses in mailing list archives. I didn't change the URL but had it served by a script instead of being a flat file. When it first went online, all of the robots kept crawling the files over and over. I finally made it supply the right mtime to Apache, which then did the right thing with the incoming IMS header, generating a 304 and not sending out new content.
It's possible this has regressed, but I would hope it hasn't.
I ran into this by chance when writing a wrapper to obfuscate e-mail addresses in mailing list archives. I didn't change the URL but had it served by a script instead of being a flat file. When it first went online, all of the robots kept crawling the files over and over. I finally made it supply the right mtime to Apache, which then did the right thing with the incoming IMS header, generating a 304 and not sending out new content.
It's possible this has regressed, but I would hope it hasn't.