I doubt Monaco is really under threat, this is traditional sabre rattling as Bernie is probably being offered a lot of money to go somewhere like russia (now petrov has been playing for a bit).
Monaco really is the jewel in the F1 crown (as the phrase goes) as its the only track where Bernie can get people who want to pay into F1 to come and watch, but who arnt actual fans. Apart from the prince of Bahrain (who despite having an average track and no local fans) is a nice bloke and F1 enthusiast, most of the new world track owners havnt much of a clue about F1 other than the money it brings. Without Monaco there isnt a place where these sort of people can be hosted and buttered up for money. Id use the British GP as an example, most the people on the grid where either famous from England or actual F1 fans (who happened to be famous, take Patrick Stewart who now is the best person ever for being big into F1), so it would be foolish for Bernie to try and drop Monaco as its the focal point of the non racing part of F1 (and still popular with the fans too)