According to this story at ESPN.com, the NFL owners have decided that tackling is too likely to cause injury to their highly paid athetes. Beginning in the 2009 season, a player will be considered "down" when a member of the opposing team touches him with two hands between the knees and shoulders. Knocking a player to the ground will result in a fifteen-yard penalty and an ejection.
NFL Commissioner Roger Goodell, when asked about the wisdom of the rules change, responded, "By the middle of the second quarter, most of our fans are too intoxicated to know what's happening on the field of play." He continued, "Fans only care that their favorite player is in the game, not what that player is doing."