http://www.apple.com/uk/imac/performance.html
"New Intel Core i5 and i7 processors.
Up to 1.7x faster at the core.
New quad-core Intel Core*i5 processors are standard on the new iMac. Choose a 21.5-inch iMac with a 2.5GHz or 2.7GHz quad-core Intel Core i5. Or expand your view with a 27-inch iMac featuring a 2.7GHz or 3.1GHz quad-core Intel Core i5. You can even configure iMac with the
fastest quad-core processor available — the Intel Core i7.
All models use Intel’s recently refined chipsets. That means the new iMac performs nearly twice as fast as the previous generation,1 adding a new dimension of speed to everything you do. And since Mac*OS*X is designed to take advantage of each core, it captures every last bit of performance from the processor."
Surely this is false advertising...