I think I have found out what was mysteriously caching pages while I was editing drupal entries which I blamed on site5. It appears that Microsoft ISA firewall is caching pages, even if your proxy settings are not set up to use it. It also appears that it does not cache very well, if you look at a page, go to edit it, then return to the page, ISA serves up the old version from the cache and you have to refresh the page to see any changes. It's cache settings have a crude option marked 'Cache dynamic content (objects with question marks in the URL)' but I have nice url's set up so that doesn't do me any good.
I have set it to 'frequently expire' cache entries.
Moral: use squid which does not appear to have the same problem.
Peter's Moblog
Seems you re-discovered the wheel. Simply reading ISA documentation would have make clear tha ISA is chaching by default even "secureNAT" clients. If you don't like this behaviour you simply define one or more "cache rules"