Testifying before Congress today, Homeland Security Secretary Janet Napolitano announced that the details of what would be necessary to apply for deferred action will be available August 1.
The date in which the actual application can be submitted to USCIS is likely August 15, 2012. Everyone who believes they are eligible for deferred action should try to prepare in advance.
It is also important to keep in mind that applying affirmatively carries a risk: you could be put into deportation proceedings. Therefore, it is extremely important that you consult with an attorney before applying.
You should be wary of any attorney or representative who urges you to apply without explaining the accompanying risks. Good luck!