RapidSSL one is old and used to work on just ip.bitcointalk.org
Positive SSL is a wildcard SSL which will work on all subdomains.
The * (wildcard) allows the certificate to be used on any subdomain belonging to the yourdomain.com domain name."
For example, a Certificate application for:
*.yourdomain.com can be used to secure:
www.yourdomain.comyourdomain.com
secure.yourdomain.com
webmail.yourdomain.com
owa.yourdomain.com
anything.yourdomain.com