Aside from the multi-service support (Yahoo specifically), tabbed windows is the one thing that keeps me from abandoning Adium for iChat. Having two or more conversations going in iChat quickly eats up screen real estate, even on larger displays. According to Thinksecret, there may be hits of a tabbed iChat interface in the recently seeded developer release of Mac OS 10.4.3. Looking deep within iChat's resources, some developers have found tab images buried away. Also, suggestions that document sharing - allowing two chat participants to edit a single document simultaneously - are also lurking about, in the form of resource images and localizable.strings files.
Of course, this is all rumor. The document sharing sounds cool, but I'd be happy if the only change to iChat was a tabbed interface. That's all I want, Apple!