As far as I can tell (given that they haven't actually released anything yet, I can't verify), they will release the source under a BSD style license, which grants you copyright rights to modify and use the source as you like. A BSD style license grants you no patent rights, however. They will furthermore release a binary blob that grants you patent rights, but only if you use that specific binary blob obtained from them and don't redistribute it.

That's as open as you can get with a patent-encumbered codec. I'm not sure what else they could do.