Webkit baked into Aperture
The kind Webkit loving folks at Surfin' Safari tell us that Apple's newest prosumer application, Aperture, uses Webkit in some way, though they won't say how. Webkit is the magic that makes Safari work and is available for inclusion in any other application on OS X (which makes creating browsers very easy). Most people seem to agree that Aperture uses Webkit for the 'web journal' feature, which only makes sense.
Surfin' Safari is also collecting a list of applications that make use of Webkit, so if you are a developer with an app that uses Webkit let them know.