Instead of the new V-8, GM will use upgraded versions of their current 3.6L V-6 with direct injection which make nearly as much power as the V-8 (304hp vs 320hp) and weigh 150-200lbs less. In addition GM also has the small-block V-8 as used in the Corvette and the CTS-V which makes over 400hp, weighs less and has a smaller package size than an overhead cam engine. GM also has a new 4.5L V-8 and the 2.9L V-6 diesel which will help it achieve a 35mpg fleet average.
[Source: Automotive News - Sub. req'd]