Wizards of the Coast has posted up PDF files containing both the quick start rules and the Keep on the Shadowfell introductory adventure. Wizards is calling this the D&D Test Drive and I think it is an amazing way for players to get a feel for the game. In addition to the two PDF files, Wizards is reminding everyone that the Character Builder is available for free to create characters of levels 1 - 3.
I should also mention that the Character Builder was just updated to include the Arcane Powers supplement. Really like working with Character Builder, and it seems that Wizards is taking the product serious as the updates are coming each month (last Tuesday) on schedule.