For years we've been threatened with the promise of a fifth Indiana Jones film. Today, we can step out beneath that shadow of oppression. Today, is our Independence Day!

Producer Frank Marshall got to talking with Collider about the likelihood of having Harrison Ford fight Nazis in Atlantis or whatever, and though his response was fairly measured, he did make it seem highly unlikely.
"I say, for me, [Kingdom of the Crystal Skull is] the last hurrah. I know that yes, we talk about it, but there's no idea, there's no MacGuffin... [Lucas] isn't [hungry to do another Indiana Jones]."

Lucas recently retired and left his role at Lucasfilm to Marshall's wife, Kathleen Kennedy. He doesn't seem to be working on an Indiana Jones script at this time. Let's pray he doesn't read my Atlantis joke above and get inspired.