It seems that Apple will indeed unveil iPhone OS 4.0 at its January 27 event in San Francisco, and the folks at Boy Genius Report have dug up some dirt on the update.
According to the site's "trusty" tipster, version 4.0 will bring a slew of changes -- some of which offer further evidence of a pending tablet device. Among the new features is support for multitouch gestures OS-wide, several new ways to run applications in the background, as well as many graphical, UI and navigation changes.
The source claims that the iPhone OS 4.0 will only be available for the 3G and 3GS, and it will put them "ahead in the smartphone market because it will make them more like full-fledged computers" -- more than any other phone available. Also, there is talk of a new syncing ability for the Contacts and Calendar applications, but no other details were given.