Welcome to Player Attack!
The exploit was posted to the official Diablo III forums, by a concerned gamer who was "unable to write a support ticket". Described as "pretty much gamebreaking", the "God" mode reportedly only works for the Wizard class and disables a few flashy effects, but gamers can otherwise play any mode they like and reap the rewards in just a few steps.
Step 1: Select Teleport - Fracture. Bind it to a key
Step 2: Select Archon, tested with Improved Archon
Step 3: Hover your mouse over or near your charcter
Step 4: Press Teleport
Step 5: QUICKLY(!) Press Archon
Reportedly, the Diablo III exploit is a matter of careful timing, and gamers are reporting mixed results as to whether or not it actually works.
In most multiplayer games, this would be annoying, to say the least. For Blizzard, however, it poses a bigger threat: If gamers can't die, they can run around and pick up more loot. When they have more loot, they can take it to the Auction House, and sell the extra items for real world money.