Nexus One was going to be available to most carriers in USA, if not all. But as we all know that never happened, which could be part of the reason why it didn't sell so well. Of course the fact that it was sold online only might have something to do with that, too. Now, Google (and Samsung) is trying a different approach with Nexus S.
They are going to make Nexus S available on at least 2 carriers it seems - T-mobile and Verizon. It appears that Verizon will get the CDMA version of Nexus S, too. This appeared at a Nexus S contest:
The currently available Nexus S is a GSM device and is not compatible with CDMA networks such as Verizon and Sprint.
This suggests that the current version of Nexus S might not be the only one, and that a CDMA one is coming soon. If we're lucky we might even see a LTE chip inside if it ever launches, and why not? I'm sure Verizon will want as many LTE phones as possible to promote their new more expensive 4G data plans.