Over at "The Hamlet Weblog" the author "read a rumour about this over the weekend" (hmmm, I wonder where he read it?) and dug up an actual press release from the Royal Shakespeare Company. His theory, unfortunate though it may be, is that they're really just talking about the Theobald version which has been known about for quite some time. This is a script from the 18th century called The Double Falsehood which was "revised and adapted" from the Shakespeare original. A little more googling found me this link on Shakespeare Apocrypha that describes the play thusly: "this was initially regarded quite skeptically, but is now being looked upon more favorably following recent analysis and research, beginning with Stefan Kukowski in 1991."
I also found a blog post from June 2006, stating that the RSC has listed Cardenio among the complete works to be performed back then. So now I'm not really sure what the "new" thing is anymore.