oh also... somethign else i've thought about a lot in relation to the
"archive format" i described are "virtual" folders: a virtual folder is a
subset of the index which is presented via IMAP as a "real" folder.
(there was a DEC research mail system which did this ages ago, i don't
remember its name now.)
there are many uses for this:
a tool could perform a sophisticated search that's not supported by IMAP and present it as a virtual folder so that any IMAP MUA can see the results.
for an archive containing 100Ks of messages you could create date subrange views again so that any IMAP MUA can read it without performance difficulties. consider the application of this to public mailing list archives: an archive mailbox presented via anon read-only IMAP so that you can browse it directly with your favourite MUA (inluding webmails which talk IMAP for a google indexable web interface).
there's technically only the need for a single index and collection of mailboxes (for each user) -- delivery filtering tools (i.e. procmail/maildrop) could place the message into one of several virtual folders which the user treats as inboxes.
-dean