It'll be a BIS-B Push based handheld client leveraging RIM's past app experiences and making use of the new components being developed within MySpace projects.
It will feature core BlackBerry application integration, and be tied in to the address book, browser and device setup wizard. You can expect the user interface to be along the same lines as other RIM-built social networking apps and offer a similar experience. As for features, it'll have everything you'd expect a twitter app these days to have, including trending topics.
If true, the news could be a slight concern for market leader UberTwitter, which might lose its lead after the release of an official RIM app. Certainly, RIM could integrate the app much better with the BlackBerry's core features. That said, UberTwitter and TwitterBerry have been given a long time to perfect their apps and develop fully fledged feature sets.