We have implemented a Single Sign-On system for SDN, SPN, Personal Portal and License System. It means that you only have to log in once on any of the sites to be able to access the rest. The side effect is that all remembered passwords are expired sooner than they usually do, which I hope is a minor inconvenience.

The solution behind it is very low-tech: by setting the cookie domain to ".sitecore.net" (note the leading dot) we can share the same cookie accross multiple subdomains.