There seems to be some question about the application of that UCI rule and lots of ambiguities. Even Contador's manager (his brother) has said they'll have to look at the situation because the interpretation of the law is confusing.
This article from Bicycling discusses the situation, but this certainly is an interesting question.
While Contador is clearly the biggest name affected, Astana had tried to keep a number of riders (including Zubeldia and Popo) who wanted to go to Radio Shack. So this could end up affecting a few people and teams. Quick Step has apparently made a big push recently for Contador as well, and reportedly offered to let him bring along 4 riders with him. It will be fun to see how this all plays out.