First things first - two brand new DLC packs will launch on Thursday February 14th. The first is a freebie, and includes the Peugeot Flux as well as two new game modes. Here are the full details:
Free Challenge Pack (Free)
"Download the new Free PGR 4 Challenge Pack from Bizarre, and you'll drive the hydrogen-powered 2007 Peugeot Flux, winner of the 2007 Peugeot International Design Competition. Plus, you'll get two new game modes-one of them the player-requested Tourist Mode, which lets you tool around any city in the game checking out sights with your friends. Or play Free Roam Cat and Mouse, where a slow mouse vehicle must avoid enemy cats. And you add another 10 achievements. Get more racing fun for the best price-free!"
The second variant is the Premium Challenge Pack. This one includes everything that's in the free pack, plus a bunch of new vehicles (cars and bikes), and a new World Challenge Arcade Mode. It'll set you back 400 MS Points, and includes all of this cool stuff:
Premium Challenge Pack (400 points)
"The Premium PGR 4 Challenge Pack from Bizarre, available for 400 Microsoft(r) Points, gives you seven new cars-the 2008 Chevrolet Camaro Convertible Concept, 2007 Volkswagen Golf GTI W12-650, 2007 Peugeot Flux, 1987 BMW M3 E30 DTM, 2007 Ferrari 430 Scuderia, 2008 Honda S2000 CR, and 2008 Callaway C16 Speedster. You get three new bikes-the 2007 Yamaha YZF-R1, 1995 Aprilia RS 250, and 1998 Aprilia RSV 1000 Tuono R."
"Plus, new World Challenge Arcade Mode gives you 20 arcade events in 3 themed chapters, including 5 events that can only be done on platinum. In Tourist Mode, get a free ride with friends around any game city. In Free Roam Cat and Mouse, a slow mouse vehicle runs from enemy cats. Ten new achievements fill out the pack."
More here including screenshots, new achievements, modes looks good.

http://www.bizarrecreations.com/article.php?article_id=5301