<p>I have a fairly simple addition to the HTTP standard.  An ambitious goal I know, but I'd at least like to submit a proposal and get feedback on the idea.  What is the proper forum/method of doing so?</p>

According to W3C's
HTTP page
:
Now that both HTTP extensions and
  HTTP/1.1 are stable specifications,
  W3C has closed the HTTP Activity. The
  Activity has achieved its goals of
  creating a successful standard that
  addresses the weaknesses of earlier
  HTTP versions.
An effort to revise HTTP/1.1 started
  in the
IETF httpbis Working Group
The page from the latter link has a mailing list you can join, as well as the scope of the working group. You may well be able to get feedback about the merit of your idea in the abstract, but certainly don't get your hopes up at effecting change.