Mark,
All D&D Next playtests are under NDAs. Open or Public is not the right word for it. General Playtest is a better term for it.
As I understand it (remember I am just a volunteer playtester and not someone running the show with full details of the program) that playtesting is broken down into to two categories:
1) Friends and Family Playtest. This group is the already established 4e playtest groups, WotC employees (with friends and families), and anyone else they want to do established testing. These folks will get stuff first, probably a larger sample to test, and their feedback will help smooth some of the rougher edges of the testing material.
2) General Playtest. This is what everyone that hit the big red sign-up button is getting into. This will be a massive group that will stress test specific material to really catch all the weird stuff that Friends and Family missed. They will also soliciting ideas for incorporation into the edition. See something that isn't the best representation of D&D, explain why and propose something else.
Now, this is party from my experience and reading what has been put out there about these playtest types. However, they could other levels of playtesters that i am not privy to and of course I could have misread some of my info, so take this with a grain of salt.
I hope this helps,