Diablo 3's patch 1.0.5 is now live on the public test realm with a new level 60 challenge called the Infernal Machine. A few players have explored the PTR and reported exactly what you need to do in order to activate it.
In order to use the Infernal Machine, you'll first have to build it. The four components for the Machine can be found on special monsters at the following locations (via Diablo Fans):
Once you've built the Infernal Machine, head to the Heretic's Abode in New Tristram and activate it. The Machine will then open a portal to one of three realms. Each realm features a different encounter involving two bosses from elsewhere in Diablo 3:
Experiencing all three fights could take a little while, then. Fortunately there are some excellent rewards up for grabs. In addition to normal loot, you can earn components for legendary Hellfire rings. You'll need three of these parts in total in order to make a ring. The plans are sold by Squirt the Peddler in Act 2.
The Infernal Machine is one of two main features in Patch 1.0.5, the other being Monster Power. The update is currently undated but you can try it out for yourself by downloading the PTR client of the game.
Update: Patch 5.1 is now live.