I have been having this same problem.
I encrypt a portion of text with a friend's public key. Then I sign the entire email (which is just the encrypted output). However, when he gets the email, the signature worked, but the decrypt is not valid. I think I have found what the problem might be. After doing the signing the encrypted portion has 2 extra '-'s.
Example:
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
- -----BEGIN PGP MESSAGE-----
Version: GnuPG v1.4.7 (MingW32)
Comment: http://firegpg.tuxfamily.org
[encrypted text]
- -----END PGP MESSAGE-----
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.7 (MingW32)
Comment: http://firegpg.tuxfamily.org
[signature]
-----END PGP SIGNATURE-----
Notice the extra '-' at the BEING PGP MESSAGE and END PGP MESSAGE lines. I think that is what is causing the decryption issue. If I copy that block, save it to a text file, take out the extra '-' and then decrypt via the command line, all is good. The workaround isn't terribly difficult, just time consuming.