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When sending a GPG-signed message using Pine (via the gpg-sign filter), the content-type is 'text/plain' rather than 'application/gpg; x-action=...'. This means that if the receiver is using something a real mail client like mutt ( ;-) ), they have to use procmail to fix up the content-type first (or do it by hand). To reproduce: send mail with pine and sign it; read the mail using mutt.
This is not a bug in Pine. Pine allows you to change the content/type of a message. I would say the bug is in the gpg-sign filter or the mistake is in the way you use it. Did you use the _MIMETYPE_ token when sending the mail?
I've read the relevant RFC's, and I agree that this is something that should be fixed. I don't consider it an extremely high priority though because the pinegpg app has a few other problems and I am going to try and find a better replacement for it if there is one out there. If not, I will look into this again some time before too long and fix it. .
Going through deferred items. pine is deprecated now, so closing this as WONTFIX