On the Development Seed blog today there's a new post by Alex Barth about integrating OpenID support into your Drupal installation. You can check out an example of it in this github project.
After a survey of available single sign-on solutions [for a client], we decided to go with an OpenID based approach since we needed to support different domains, wanted to avoid sharing user tables and did not want to add complex system requirements for browser clients or the server. [...] The great advantage of this scenario is that we know which five sites need to play nicely together and all of them are Drupal sites under the client's control. This premise allows us to add an additional site as a designated OpenID provider that we call "Hub" and make all five sites point to the Hub as their default identity provider.
They use two modules to make everything work together - OpenID SSO and OpenID Provider SSO and a PubSubHubbub model for keeping the user information up to date.