Depending on model, the new iMacs ship with Core i5-2400S (low-end 21"), i5-2500S (high-end 21" and low-end 27"), i5-2400 (high-end 27") CPU's in their standard configurations. BTO variants are available with Core i7-2600S (high-end 21"), and Core i7-2600 (high-end 27").
I actually didn't know the iMac used all low-power Sandy Bridge designs ("S") for their CPU's except for the high-end 27" model (both in its BTO variant and not).
All CPU's here were introduced in early 2011 and I believe at the same date as the 2500K. 🙂