Unfortunately, you can't guarentee that at all; the Expires header is defined in the standard, and therefore, any proxy should follow it, but as with users and homegrown browsers, they don't have to. I know that in recent discussions on my isp's newsgroups on the possibly of installing a proxy, most proxies that are used at a large scale site are home grown and typically have had much trouble with 80% of the web sites out there that *don't* follow the standard.
Again, falling back on a sessionid and other tracer should help prevent problems from cached page use.
Dr. Michael K. Neylon - mneylon-pm@masemware.com
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"You've left the lens cap of your mind on again, Pinky" - The Brain
