SIP appears to be ideal for use in persistent MMO games. It supports real-time data session set up and Instant Messaging. There are already two SIP open standards for instant messaging XMPP and SIMPLE.
SIP Presence information can also be used to see who's playing and invite other users to join a game, instead of blindly logging on to a games master server. Players just has to subscribe to the presence information of friends. This presence information can be displayed in "buddy list" format showing who is playing, who is online and available, and who is not. Third party call control could even automatically initiate a gaming session when everybody was ready.
The more I read up on it, the more promising SIP looks. What I find interesting about SIP is that I was already implementing home-brewed protocols to set up sessions and handle real-time simulation data messaging. I do prefer to use open standards. SIP looks real for MMOs, but, sure any type of online game could benefit. I'm suprised there hasn't been more discussion on this topic.
