I assume the iPhone 4 will be available on Sprint/Verizon/AT&T. Since the 3GS was exclusive to AT&T I can't imagine Apple upgrading it to be compatible with CDMA carriers. It'll probably remain on AT&T exclusively.
they might have the 4 for sprint too. Sprint and Verizon should be able to run off the same hardware, so it'd only be a software tweak to get the 4 to run on Sprint