Ok, guess what? The Mail.app importer discards the "Date:" header and uses whatever was in the envelope ("From_") instead. So, not only is this bogus (the send-time is more interesting than the receive-time) but it means that it's going to mis-date just about all of my really old mail, since those have correct Date: headers but seem to have in their envelope the date at which they were converted to "mbox". I blame Kyle Jones.
This means I get to write a script to parse all those Date: headers and regenerate the envelopes. How many times have I done this? Then re-import the mail again. How many times have I done this?
otterley: I gave up on IMAP because the server is just far too much of a pain in the ass to figure out how to get running.