Multiple Wordpress Sites, Need Single Sign-On

We have several new sites that are like this one: http://select*******Remove*this********[login to view URL] They all have simple-press forums. Need to have a way for visitors to log-in on any of the sites, and then be able to comment on any other site without having to log-in again at any of the other sites (single sign on). Please let us know basically how you would do this.