How about an extension to Grand Central Dispatch that would make it easy to distribute processing tasks not just between multiple cores, but between huge numbers of computers? We've already got something like that in the form of the cross-platform BOINC platform, so that would hardly be something "totally new".
But you could argue that would be totally new in the same sense that "The new Magic Trackpad is the first Multi-Touch trackpad designed to work with your Mac desktop computer" ignores the products put out as far back as 2005 by FingerWorks (subsequently acquired by Apple). That is, the term 'Multi-Touch' wasn't used to describe the functionality and the device was designed for cross-platform use rather than primarily for a desktop Mac.
The job ad implies that the successful applicant will be working on something that will "be used every day by millions of Apple customers" so it needs to be something fairly central to the Mac experience.
We'll find out soon enough. When Steve Jobs is on stage presenting the features of 10.7, maybe we'll look back and think "oh, that was what that job was about."
If you're interested in applying, the job description is here.
But does it really have anything to do with 10.7? Please read on.