According to a post on Shacknews , it seems that id Software has officially announced that they are starting development on DOOM 4. Although no specifics about what the game would entail were given, id Software CEO Todd Hollenshead did give a little clue saying:

"Doom is part of the id Software DNA and demands the greatest talent and brightest minds in the industry to bring the next installment of our flagship franchise to Earth".

Hopefully that means the sequel to DOOM 3 will take place on Earth with the hordes of Hell invading the planet like the original DOOM II did.