[speleding]

Interesting. I looked at the RFC and it says it has to be in the format "Sun, 06 Nov 1994 08:49:37 GMT" (RFC2616/RFC1123).

But even if browsers makers would check the "If-modified-since" against that format then it would still be doable to give each visitor a slightly different date and track them that way.

Combining the date stamps on 2 or 3 jpg or css files present on every page on the site should give you enough entropy for even the highest traffic sites and make it very hard to detect.