> I was wondering why the *Update option causes the mirrored
> site to be entirely re-downloaded?
Maybe because the remote server was not able to fulfill the 
update request, probably ?
> The pages are not served up, they are static.

Beware: some "static" contents are sometimes dynamically 
generated (such as html, to interpret embedded php 
scripting)

> There is a javascript command:
> javascript:alert(document.lastModified)

Not useful for an engine like httrack - the javascript 
layer is on the top of the HTTP layer. 

> We don't want them to go away. I'd hate
> for my periodic mirroring to cause the sites I love to go
> away due to bandwidth consumption!

Try to track (Experts Only / Debug mode) the 
requests/reponses if you know how to read HTTP 
requests/responses (hts-ioinfo.txt) ; you should see update 
lines (If-Not-Modified-Since: and optionnally If-None-
Match: lines)