As usual, the release date for any Blizzard production is kept under wraps for as long as possible. What is known is that Diablo III is behind StarCraft II in the development cycle. A StarCraft release for 2009 and a Diablo III release in 2010/2011 seems probable. The demo shown to players was very well polished, which makes it harder to estimate development time left. Any other dates or rumours are that -- rumours.
The latest art direction changed about a year ago [1] (said on GC 08 byt Jay Wilson), and considering how far they have come with general western Sanctuary landscape and dungeon interiors, that gives an indication on a cycle of class balance, and monster/terrain creation for an additional 2 or so years to finish the game. More info will be available at BlizzCon 2008.
You can read more about the system requirements, beta testing, price/payment methods and age rating on the Diablo 3 Basics page.