Courtesy of Motor Trend recently:

General Motors' Cadillac-only Sigma platform (CTS, SRX, STS) proved too expensive for cars like the Pontiac G8 and Chevrolet Camaro. Now GM is merging the Sigma architecture with its other RWD platform, the G8/Camaro's Zeta, in time for the urgent 2011-model-year Cadillac STS/DTS convergence. Cadillac has locked in the new flagship's design. We hear designers are reworking the nose because the flashy headlamp design turned out to be too busy.

The converged Sigma/Zeta platform will encompass six versions on three wheelbases:
Coupe: Next-gen Chevy Camaro, although Holden is still pushing hard for a Monaro version, and Pontiac may want a G8 coupe, too.
Coupe premium: Next-gen CTS. Same wheelbase, but gets forged-aluminum suspension for premium road feel, handling.
Midsize: 114.8-inch wheelbase, including Pontiac G8/Holden Commodore and possibly a Chevy sedan, but not the next Impala.
Midsize premium: Next-gen CTS. Also shares 114.8-inch wheelbase (a 1.4-inch stretch over the current car), but gets the upscale forged-aluminum suspension.
Large: 118.5-inch wheelbase for Holden Statesman/Buick Park Avenue (China), Holden Ute/Pontiac G8 ST, and maybe RWD Lucerne replacement.
Large premium: New STS/DTS replacement. Same 118.5-inch wheelbase as the large sedan, but with forged-aluminum suspension and longer overhangs.