Ginsberg and Roberts have not written an opinion since May 21. That means they are probably the authors of the Majority opinion and the Dissenting opinion. If that is the case they are probably on different sides of the issue.
I would suspect that makes Mr. Tapper’s claim improbable.
We will know for sure shortly.
http://hotair.com/archives/2012/06/25/oh-my-is-ginsburg-writing-the-main-obamacare-dissent/
I guess Jake Tapper has pretty good sources. The Democrat-appointed members of the court don’t stray. Those appointed by the GOP frequently do (Stephens, Souter, Kennedy, Roberts).