In the first days, the PGP sign was included inside the text to sign. Later, it was included the application/pgp MIME type to show that the next attach was the sign or the encrypted PGP message, and finally, with the PGP/MIME specification, it was possible to isolate the sign from the original affected, to not modify absolutelly and somebody that didn't have PGP could view the message as it was originally (only for signed messages), without any added text in the beginning or in the end from PGP.
The actual situation is that only a few mail user agents (MUAs) are capable to integrate PGP to use the PGP/MIME standard, and it is necesary to send messages using the old time PGP sign when you know that the recipient doesn't recognize PGP/MIME.
In Linux, the available mail user agents that are PGP/MIME compliant are mutt-i and pine. In Windows, only the Eudora mail client versions 3.x and 4.x can use PGP/MIME. If you know any other mail user agent that supports it, tell us by mail, to include it here.