A while ago I tried to sign up for a passport account using an e-mail address other than a Microsoft provided one, and seemed to end up nowhere. Here is a link that will allow you to register a non Microsoft provided e-mail account for the Passport service. Apparently that was only a temporary thing.
This is good if you want to have your RMS environment trust Passport RACs, so you can send content to people outside your organization.
Remember though...by trusting the passport service, *you are allowing people to send RMS protected content outside your organization*.