Hot from the beta forums, a blue post posted 7 hours ago
We will soon begin public testing of patch 5.0.1 of World of Warcraft, a patch that will precede the Mists of Pandaria release, and includes many features and functions that are new to the game.

We encourage all players who wish to test patch 5.0.1 with us to take careful note of the following:

It will not be possible to access the 5.0.1 PTR realms with a game account that accesses the Mists of Pandaria Beta.
Access to 5.0.1 PTR and Mists of Pandaria Beta - Forums - World of Warcraft

On Cata they did that around 1-2 Months before launch date.