I just wanted to confirm this. If you compose in plain text mode, the signature will be formed from the unwrapped version of your message causing subsequent verifcation to fail. "Rich text" mode appears to work, because no wrapping is needed there.
In this example, _sent_ contains what I saw in the message window before sending, and _rec_ contains the body of the same email once received.
ben@dev:/tmp$ cat sent
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.6 (GNU/Linux)
Comment: http://firegpg.tuxfamily.org
iD8DBQFGN0u+VsSLShHy9OARApHnAJoCZ22eAdEQ742SS87/rG7DDOVTggCfQZmP
AVxffs3ekhHLYozyd75ayKM=
=ZNVZ
-----END PGP SIGNATURE-----
ben@dev:/tmp$ cat rec
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.6 (GNU/Linux)
Comment: http://firegpg.tuxfamily.org
iD8DBQFGN0u+VsSLShHy9OARApHnAJoCZ22eAdEQ742SS87/rG7DDOVTggCfQZmP
AVxffs3ekhHLYozyd75ayKM=
=ZNVZ
-----END PGP SIGNATURE-----
ben@dev:/tmp$ diff sent rec
4c4,7
< aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
---
> aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
> aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
> aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
> aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa