I believe, that this patch: http://bugzilla.gnome.org/attachment.cgi?id=84585 fixes this bug. It is still unconfirmed, not accepted and not revisioned by the upstream maintainer (no maintainer responded to my posts on Gnome Bugzilla for over two weeks) but patched version seem to work well at least for me and my workmate. Please test it.
Here is a link for whole entry on Gnome Bugzilla for this issue: http://bugzilla.gnome.org/show_bug.cgi?id=368877
smime.p7s
Description: S/MIME cryptographic signature
