Single Sign-On Between DotNetNuke site and Drupal site?

What are good approaches to consider for implementing Single Sign-On Between a DotNetNuke site and a Drupal site? They will be on different servers and use different languages (.NET and PHP). I want a visitor who is logged in at one site to be automatically logged in when they go to the other site.

I don’t even think it is possible to do a cross website login for security concerns, and browser behavior (change in URL path). That being said, you could use a cookie, but I don’t think it is possible.

This could work in IIS7 due to the nature of the beast. There was an article or two on using .NET auth with wordpress, that is where I would start as it is a pretty similar problem.