A new difficulty level has been confirmed in Diablo 3, inferno! It is aimed at players who reach the level 60 player cap, featuring monsters level 61 or higher. It sounds challenging, but the unique drops should keep you coming back for more.

Don't worry about doing the same runs over and over again, though. The drops should be more evenly spread out, and the high level monsters more diverse.
Bashiok, a Blizzard employee posting on their forums had the following to say about it:
Primarily it's focused around Inferno and creating a consistent difficulty across the entire act. By having a flat difficulty across the entire game in Inferno it evens things out so that no matter where you want to go you're being challenged, and you have a good chance at getting good drops. That's intended to open the game up so there's simply more freedom for the player to go anywhere in the game and have an equal chance at good drops.
There will probably still be a slight difficulty curve in Inferno, as enemies simply become more complex and have greater abilities later in the game, but we're making an effort to make it as even across the board as possible. It's something we'll very likely have to keep tweaking after release.
In addition we're attempting to de-emphasize path of least resistance by creating a more equal value across monster types. We're not going to get into specifics about who can drop what or how much, but our intent is that being out in the world exploring should be as lucrative (potentially more) as simply aiming for the quickest-to-reach boss and running that over and over.
Getting people out into the world of the game and away from repetitive boss runs is essentially our goal. It's going to be tough because people will find a path of least resistance, and they will make that the game. We're going to have to keep on top of it, and truthfully it's something we're still working on. We'll likely want to have some additional features to support these ideals, but just getting Inferno tuned right is our current end-game goal for ship.