the current cts sts & srx are on sigma
the new '08 CTS is gonna be on sigma II along with 2009 sts & 2010 srx
some good info here.
'A so-called “Sigma Lite” variant might reach production in 2007 as a replacement for the reworked Zeta platform. These vehicles would be built with less-expensive components and would be sold by Buick, Chevrolet and Pontiac."