The addition of a Yahoo! sync framework in 10.4.10 certainly seems a bit puzzling on the surface, but after some grinding of my mental cogs, I think I've figured it out.
If you will think back to MacWorld 2007 and the announcement of the iPhone, you'll remember that Steve Jobs brought three special guests on stage. Stuffed between Eric Schmidt and Stan Sigman (of Google and Cingular, respectively) was Jerry Yang from Yahoo! who used his time to talk about the free Yahoo! IMAP "push" email that will be bundled with every iPhone.
With an email account comes a contact list, and in this day and age of do-everything service providers, a calendar usually comes with it. It makes sense then that Apple would make it possible to edit and sync these things from your desktop– thus the new framework. Sure, I can do all that from my iPhone, but do I really want to enter and tweak 300 address book entries on such a small device?
Obviously, this is just idle postulation, but I'd be surprised if something like this didn't get announced before the release of the iPhone later this month.