There have been numerous rumors that there may be a follow-up to Indiana Jones and the Kingdom of the Crystal Skull. Yes, another Indiana Jones movie may be in the works.

Says who? George Lucas himself!

The director spoke to ComingSoon.net about the possibility:

ComingSoon.net: You had huge success this summer with "Indiana Jones and the Kingdom of the Crystal Skull." Are you seriously thinking about another one? Harrison Ford's said there may be plans afoot.
George Lucas: Yes - we're looking for a "MacGuffin," which is an object that he goes after. They're very hard to come by!

ComingSoon.net: Would you want to keep the whole family together for a film - Indy, Marion and Mutt?
George Lucas: It really depends on what it is Indy goes looking for and then how the story falls out of that, and then how convenient or inconvenient to have the group there