SSL: How are certificates protected against man in the middle attacks?

Certificates are signed by some trusted authority, such as Verisign.

The certificates for these root authorities are built right into the browsers when you download them. You can view the root certificates in Firefox, for example, by going to tools-->options-->advanced-->encryption-->view certificates-->authorities.

If any one of these root-certificate authorities is compromised, however, you are right that a certificate could be forged, making a man-in-the-middle attack possible.

No, the certificate is not encrypted. But it is signed by a certification authority (CA). Since those check the information included in the certificate (especially the URL to which the cert belongs), there shouldn't be a second valid certificate for a given URL.

The cert of the CA is checked against a trust store (e.g. in your browser). If this truststore is compromised, or if you trust not valid certificates, there is no protection against man in the middle attacks