I'm a subscriber to Comcast's High Speed Internet service and I just got an email telling me that Comcast and Plaxo have joined forces to provide the Comcast Universal Address Book. I don't have much more information other than it's free to HSI customers and it's Plaxo's Premium service that goes for $49.95/year.
I've looked into Plaxo's services before and their premium service provides synchronization services across multiple computers, multiple email clients (Outlook, Thunderbird, etc.), LinkedIn, mobile phone and more. All in all, it looks like it's a win although I'll be tieing myself to Comcast. When I have a chance to look into it a bit more, I'll post more.