From the AMA press release:
AMA Pro Racing said today that it will continue to utilize 1000cc-based equipment in the AMA Superbike Championship through the 2008 racing season.
Rules adding 1000cc motorcycles to the class were announced in 2002 for the 2003 racing season. Current AMA Superbike rules feature 901-1000cc four-stroke engines with two or more cylinders, exclusively.
This means continuing development of those bikes from all of the manufacturers contesting the series and ever faster bikes on the showroom floor.