How does 37signals code their "Open Bar" (Open ID) feature?

See here:

Say you have two applications:

1) Basecamp
2) Highrise

How would Highrise know that the user (with the same OpenID credentials)
exists on another application (Basecamp)?

Is there something involved for that to happen? Are they using
ActiveResource to fetch the external user table, and then searching for
that OpenID?

Go here,