Yes the long awaited sequel is coming. Harrison Ford is signed on, Steven Spielberg is signed on, and of course George Lucas is holding things up!
A script was written that featured Kevin Costner playing Indiana Jones brother. George Lucas scrapped the script calling for re-writes so the screen play is still being written.
Also rumored is Karen Allen (Marion Ravenwood), Sean Connery (Dr. Henry Jones), and Natalie Portman.