> I had seen ETags mentioned previously, but no solid
> documentaion on how to use them with httrack.

Well, the mechanism isn't documented in httrack, because it 
is already described in RFC 2616 
(http://www.ietf.org/rfc/rfc2616.txt)

The idea is to implement a mechanism that will allow to 
check the "freshness" of a ressource (identified by the 
full URL - INCLUDING the query string), by using a specific 
server header response, "Etag". This header allows the 
server to send a string that will FULLY identify the 
ressource content. It means that, given an URL and an Etag 
value, a server is able to tell whether the ressource is up-
to-date or not.

The client, when requesting a refresh, will issue a regular 
HTTP request, giving a "If-None-Match" field with the 
previous server Etag string. The server then will reply 
either with a "Not modified" status (304), or with a "OK" 
status (200) with the full content, if the cached content 
is not up-to-date.

It is up to the server to generate the Etag string, AND to 
check the freshness of the contents.

An md5-checksum of the full output data can be used, but 
any other solution is possible (a checksum of all variables 
used to generate the page, for example)

The use is not very simple depending on the server, however.