Maintenance is already underway in North America and should be finished by 1:00pm PDT. European maintenance will run from 8:00pm PDT tonight through 6:00am tomorrow, and Asian maintenance will begin tomorrow at 11:00am PDT and run through 11:00pm. Once the maintenance is complete, the patch will begin rolling out in each region. Blizzard warns players in the Americas that global play will be inaccessible in Europe and Asia until the patch is deployed in each region.
The patch is designed to improve the experience for high-level Diablo 3 players, most notably adding the Paragon system. The system features 100 new levels for level 60 players to achieve, replete with bonuses to base stats, increased magic find and ... nifty character portrait frames. Head over to the Diablo 3 forums for the full patch notes.