If you have a Jabber Id in your Profile you can choose to receive forum notifications via mail, jabber, jabber and mail now
Jabber / XMPP is a IM Service like ICQ or MSN but based on a open protocol. Google talk for example uses XMPP http://www.google.com/talk/
About jabber: http://www.jabber.org
xmpp standard: http://en.wikipedia.org/wiki/Xmpp
You can also contact forum users via the jabber link in their profiles. The message is delivered to the Jabber-Client.
It's worth to look what jabber is. It's free, it's open, it's secure (encrypted server connection and/or encrypted messages (GPG encryption possible))
No struggle anymore to contact users in a different network. No problem with xmpp based services. And like i mentioned before gtalk (google talk) use it.
Different clients to use gtalk or other jabber networks:http://www.google.com/talk/otherclients.html
If you have any questions or problems feel free to contact me.