Version:           1.8.1 (using KDE KDE 3.4.1)
Installed from:    Gentoo Packages
OS:                Linux

KMail -> Settings -> Accounts -> Sending -> Message property is set to Allow 8-bit.

When I send a unencrypted mail, the header contains:
User-Agent: KMail/1.8.1
MIME-Version: 1.0
Content-Type: text/plain;
  charset="iso-8859-1"
Content-Transfer-Encoding: 8bit
Content-Disposition: inline

If I use OpenPGP/MIME, the header contains:
User-Agent: KMail/1.8.1
MIME-Version: 1.0
X-UID: 48
X-Length: 824
Content-Type: multipart/encrypted;
  boundary="nextPart13526006.u7TYfUCnun";
  protocol="application/pgp-encrypted"
Content-Transfer-Encoding: 7bit

--nextPart13526006.u7TYfUCnun
Content-Type: application/pgp-encrypted
Content-Disposition: attachment

Version: 1
--nextPart13526006.u7TYfUCnun
Content-Type: application/octet-stream
Content-Disposition: inline; filename="msg.asc"

And the msg.asc itself starts with:
Content-Type: multipart/signed;
  boundary="nextPart3607352.BJaFHrHbfp";
  protocol="application/pgp-signature";
  micalg=pgp-sha1
Content-Transfer-Encoding: 7bit

KMail decode the 7bit Encoding correctly, but other Clients (Outlook Express and Outlook) not and they display for example m=E9s insted of més.