Ivy Bridge drops into LGA 1155 and the Intel 7 Series PCH is pin-compatible. It takes pennies (for Apple) of R&D just to drop this in and give us USB 3.0 support."There will be a new design of the iMac in 2013." An update like the MBA and MBP is still likely to come this year. No reason not update the iMac (and Mac mini), if you look at them, the new processors (and motherboards with USB 3) seem like a pretty straight drop-in.
I was going to get a Sandy Bridge notebook model refurbished but I am going to wait for the Ivy Bridge ones just to get USB 3.0 standard! Maybe in 2-3 months.