Emmm there was an almost 4 days open beta a week back.
But to answer your question, the game runs smoothly in single player, with settings to max on a Windows 7 PC with 4GB (DDR2) of RAM, Intel Core 2 4300 @ 1,8GHz, and a GeForce 8800GT. In multiplayer I had to lower decals (amount and quality) if I remember correctly... to retain the FPS at a non stuttering rate.
There will be a demo as there was with Starcraft II. If it is any indication, it took Blizzard almost 5 months to release the Starcraft II demo and then they replaced that with the Starter Edition. I wouldn't be surprised if they did the same with Diablo III.