What is a WoW Annual Pass?
The WoW Annual Pass is a twelve month World of Warcraft subscription which can be paid for in monthly installments.
When do I get Diablo III and the other exclusive benefits?
You’ll be able to download and play Diablo III as soon as it’s available and once you complete the one-year term, it’s yours to keep, even if you cancel your subscription. Tyrael’s Charger will be available with Patch 4.3, and beta access will go live with the beta launch.
http://us.media.blizzard.com/wow/promotion/wap/en-us.html