They're including a nice smallish RSS reader in their Mail app. I had seen it before, and it's a River of News aggregator.
And that, of course, sounds very interesting. I can't seem to find the RSS reader in Yahoo! Mail, so they still have to implement it, I guess. But I'm very curious!
Update: it seems there's a separate private beta for Yahoo! Mail. I didn't know that, but it explains why I didn't see the RSS reader. SiliconBeat has info.