BMW, Mini or Mercedes offer iPod integration already since a while. Now Apple says they reached agreements with GM, Ford and Mazda that will lead to 70% of new cars available 2007 in the United States to support iPod integration.

GM will offer the Personal Audio Link in October in their HHR car.
The integration displays tracks on the factory installed radio and the steering wheel volume control buttons are supposed to work too for the iPod.
In 2007 all other 55 models are supposed to support it as well. You can pick it up apparently at dealerships this fall for less than $160, plus installation.
More details on CNN Money and on the GM FYI Blog (Thanks Rod Y.).