Does KMail actually work this way? This would be enough to make me try it out... although my day to day WS is a windows box, so I'd most likely wait until KDE4 is available on Windows...
Offtopic to the left -- before I switched to using GMail for my personal mail, I used to use Sylpheed ( http://sylpheed.sraoss.jp/en/ ) and it stores messages in a MH format (a file-per-message design somewhat like Maildir) and it can import mbox files from Thunderbird. The client itself is very nice too, I remember really liking it a lot.
Bonus: Win32 binaries are also available.
I tried it a long time ago, but it was still really buggy on Windows...
I do remember that its interface didn't seem to be nearly as polished (being that it was GTK based).
But, like I said - with TBirds extensions capability, I really don't see myself moving away. There are only two big things baout it I don't like:
No proper 'Signature Manager' (yes, I've seen and used the Signature extensions (Signature, Signature Switch, etc), but they still require you to manually create the sigs first. This is a *glaring* lack in my opinion...
Monolithic message storage... this is still a biggie to me, but I can live with it...
--
Best regards,
Charles