Last-Modified: could be filled with this (the aggregated semantics for it is clear, I think: the highest modification date in the compound validity object). For objects implementing "dynamic" retrieval, like xsp sheets, they should set "now" and a close to zero expiration date
This would allow for requests with If-Modified-Since: set to return a small body (304), saving a lot for quasi-static GETable resources.
You might want to read the recent threads on proxy friendly cache headers. We were talking exactly about this, and I committed a few patches on the road of proxy compliance. If you feel like helping, you're most welcome. :-)
Ciao,
-- Gianugo Rabellino Pro-netics s.r.l. http://www.pro-netics.com
