I think it would be cool if you could put some sort of Indicator next to someone's HT Profile page that shows that they are signed into HT and on the website. This is not necessarily to show who is on XBL, but more to show who is on Halotracker.com.
OpenID is service that allows you to log-on to many different websites using a single indentity. Find out more about OpenID and how to get an OpenID enabled account.