Hi bernd1,
All this certainly is confusing, and this post only addresses the issue of CDMA for the iPhone 4S, not for any other phones or the carriers in general.
In regards to the iPhone 4S, the issue is phone compatibility. The Unicom version can only run on the following networks (UMTS/HSDPA/HSUPA/GSM/EDGE), while the Telecom iPhone is CDMA compatible. The Unicom/Telecom links in the post offer all the technical specifications of each iPhone, just scroll past the plans to the bottom of the page.
This is also the reason that China Mobile users can't get 3G b/c the iPhone isn't compatible with their network.