Yes, IMAP is the perfect solution to the problem of
synchronizing multiple different internet client
applications.  IMAP supports persistent mail storage in
folders on the server side so that all mail clients,
regardless of location, have a synchronized impression of
what the user's current mail situation is like.  MailMan
does not currently support IMAP because it is not nearly as
widely accepted as POP3 as a mail server protocol, and
because there are a fixed number of hours in a developer's
day.  We sincerely hope that IMAP becomes more prevalent and
demand for an IMAP version of MailMan increases, and we hope
that the existing versions of MailMan will be successful
enough to support the development of an IMAP version.  If
you would personally like to see an IMAP version of MailMan
with full server-side folder support, drop us a line at
mailman@endymion.com and let us know that you are out there.
If the demand is high enough we will naturally be compelled
to develop a product.