We've been discussing several solutions for the Single Signon problem in Drupal, and got to the conclusion that this is a really confusing topic and that there are several projects trying to achieve this goal in multiple ways, some of them work, some of them not quite.
It all started with Aegir and Bakery, both are incompatible and the Bakery maintainers said Bakery will never support Aegir. It seems that the only solution was LDAP or OpenID. While LDAP could work, it is definetely not an easy option for most users, OpenID would be easier. We've been having a discussion at http://groups.drupal.org/node/140874 where we've decided to move to D.o a working solution developed by DevSeed at github. This solution, although it works, it needs work, love and lots of documentation since there are some confusing names here.
I will probably be leading the project but I will need co-maintainers. If anyone wants to jump in and help, please post it here.