According to MSNBC.com:
Carlin went into a Santa Monica hospital Sunday afternoon complaining of chest pain and died later that evening, said his publicist, Jeff Abraham.
Carlin, who had a history of heart trouble, performed as recently as last weekend at the Orleans Casino and Hotel in Las Vegas. It was announced Tuesday that Carlin was being awarded the 11th annual Mark Twain Prize for American Humor.
Only a handful of people can ever be said to have had a major impact on the culture, and Carlin was someone who not only impacted it, he changed it and redefined it.