Version:           svn-r915963 (using KDE 4.1.96)
Compiler:          Ubuntu 4.3.2-1ubuntu11 
OS:                Linux
Installed from:    Ubuntu Packages

I have installed KDE4.2-rc1 from ubuntu PPA packages, and compiled kdepimlibs and kdepim from trunk (rev 915963).

When I click on a pgp signed message, kmail shows the yellow box with the text 'message could not be decrypted', although it is only signed.

This message has a -----BEGIN PGP SIGNED MESSAGE----- header in the beginning of the message, after the headers.

Another message from the same mailing list was shown correctly by kmail, but that message used a MIME multipart, used no -----BEGIN PGP SIGNED MESSAGE-----, and has a Content-Type header:

Content-Type: multipart/signed;
  boundary="nextPart3653173.fyHFIKDB5r";
  protocol="application/pgp-signature";
  micalg=pgp-sha1