It's no secret that iPhone OS 4.0 will debut on January 27 along with whatever super-secret product Apple has hidden up its sleeve. Boy Genius has just gathered some information from a reliable contact on what will be included in the 4.0 update.
First off, there will be multitouch gestures OS-wide now, and a bunch of new ways to run apps in the background. Apple seems to be expanding both their gesture library, and the capacity of iPhone OS devices to multitask. I doubt they'll be able to compete with Android's multi-tasking abilities at this stage, but that's clearly what Apple is moving towards.
There have been a variety of UI and graphics changes, intended to make the OS easier to navigate. According to the contact, everyone in Apple is excited about these changes. The update is only for the 3G and 3GS, and it should help to make those devices act even more like real computers. New syncing options for contacts and calendar apps will also hit with this update.
Of course, we'll have no way to check on the veracity of these leaks until Apple's event. Nothing here is very difficult to believe, though. I wouldn't be surprised if these OS 4.0 leaks turn out to be spot-on.