As others have said, according to the official info you can earn AM and MPO miles / sectors on US eff. 31MAR14. You also don't have to wait till April to book. Date of travel matters, not date of purchase.
http://www.oneworld.com/news-informa..._col_count%3D5
The 140 million members of established oneworld airlines' frequent flyer programmes will also receive alliance services and benefits when flying on US Airways from 31 March. From day one, this will include the ability to earn mileage and tier status points and, for Emerald and Sapphire cardholders, to access US Airways' lounges when flying on oneworld departures, although the ability to redeem rewards for flights on US Airways, may be phased in shortly afterwards.