If i forward a message from icedove (using ctrl+L), it shows up in the
outgoing message as a new mime part with Content-Type: message/rfc822
and Content-Disposition: attachment.  I think it should be
Content-Disposition: inline.

This choice of Content-Disposition: attachment causes compliant MUAs
to not display the body of the forwarded message, which is almost
certainly not the intent of the forwarder.

Two clear examples of where this is wrong:

0) gmail users who receive forwarded mails will only be offered the
   choice of viewing the raw attachment source (with all headers) or
   downloading the "*.eml" file (which most people then have no idea
   what to do with).

1) the per-bug webpages of the debian BTS do not render forwarded
   e-mails when they are Content-Disposition: attachment, but they do
   render forwarded e-mails when they are Content-Disposition: inline.
   for example, compare http://bugs.debian.org/690135#20 with
http://bugs.debian.org/690135#25 -- message #20 was sent with
   Content-Disposition: attachment, and message #25 was sent with
   Content-Disposition: inline.

This can be worked around by setting mail.content_disposition_type to
0 in the config editor, but this shouldn't be necessary.  Users who
forward an e-mail to someone else expect the recipient to be able to
read it.

See also:

http://kb.mozillazine.org/Send_attachments_as_real_attachments

Note that Icedove actually renders message/rfc822 MIME parts in-line
regardless of whether they are marked with Content-Disposition:
attachment or otherwise.  This is a separate bug, but it masks the
misbehavior reported above when mails are forwarded between people who
are both using Icedove/Thunderbird.