erm... OpenLDAP anyone?
Seeing as OpenLDAP can be configured to fool windows machines into thinking they are connected to AD and even allow sysadmins to use the Microsoft AD utilities, isn't there already an option for SSO under windows/linux?
Couple this with CUPS, PHPGroupware/OX, Asterisk and SAMBA and you've got a complete replacement for small business server that runs on much older hardware and is far more stable...
TurboLinux - think carefully about what you are doing...
(cue MS Fanboy flames...)