Lawrence K. Ho / Los Angeles Times
Leno has said he's given his blessing to Jimmy Fallon taking over as "Tonight" host in 2014. But he said the same thing when O'Brien's takeover was announced.
Already, there are bad feelings in the air.
After NBC Entertainment chief Bob Greenblatt reportedly complained to the "Tonight" host about jokes poking fun at the network's low ratings, Leno responded last month with a holiday-themed joke: "St. Patrick drove all the snakes out of Ireland -- and then they came to the United States and became NBC executives."