Drori said that, "It is more than research. We intend to have it as part of the offering. The Whitestar can be all-electric or it can be an REV." That's pretty straightforward.
So, why is the electric car posterboy looking at hybrids (presumably a plug-in) with a smelly gas engine attached? Range, said Tesla chairman Elon Musk. While the WhiteStar EV will be able to go 150-200 miles on a charge, the REV will go further (natch) and cost slightly less. This is good because Tesla knows there is a good possibility that the $30,000-$40,0000 Volt will go 400 or so miles between charges/fill-ups. While not as expensive as the Roadster - the WhiteStar will still sell for luxury prices ($50,000-$70,000, depending on options) - getting REV technology refined on the WhiteStar will make Tesla further-down-the-road "economy" sedan have a range that can compete with the Volt and whatever other options are available at that time. Whenever that is...
UPDATE: Guess I should make this clearer, and Darryl pointed out below, that Telsa has been publicly discussing the REV option for almost two months. This is just the most concrete confirmation of those plans we've heard yet.
[Source: Michael Kanellos / C-NET, h/t to Don A.]