THIS article from Motor Trend states that the car is based on the GM Epsiilon platform, which isn't too crazy to believe since the top engine choice will be the 3.6L (like the XL7 has) also found in the 2008- Chevy Mailbu...an Epsilon car as well.
Side note, however, the MT article also says the 3.6 is a direct-injection engine...which unless something big has changed, is not the case. So they might be totally full of cr@p anyways....oh well.
THIS review from Edmunds says entirely the oposite, saying that the Kizashi is on a totally new platform, sharing nothing with GM. I guess really we'll get to know probably next month when the sedan (still called "Kizashi" or not) is scheduled to debut in New York.
~Erik~