I'm also interested in this feature.
Now i'm running thunderbird with davmail against exchange 2007 and we mainly use sso because we are not allowed to save passwords in applications (policy). Now when i start thunderbird with the lightning plugin i have to login 4 times.
One for email,adressbook,sending (smtp) and calendar. The webinterface works single sign on within firefox but i'm not sure how easy or dificult it would be to implement this.

I use this software now 2 weeks and it works!