Last night, major retailers across the United States began offering Intel Ivy Bridge processors along with Ivy Bridge-optimized Intel Z77 motherboards (Sandy Bridge H61, H67 and z68 MoBos/Chipsets are still Ivy compatible). You can even find significant discounts ($50/off at Amazon above) already.
As TonyMacx86 notes, a kernel patch is necessary to build a Hackintosh with Ivy Bridge currently. That has not stopped some savvy Hackintoshers from getting MacOS up and running (and benchmarked). However, Apple has not shipped a native OS kernel compatible with Ivy Bridge, which makes the patched kernel less desirable than a vanilla kernel that supports Ivy Bridge.
It is not certain if Mac OS 10.7.4 is Ivy compatible (commenters—correct me, if I am wrong).
With Ivy Bridge processors now on store shelves, it would seem that there are not any external barriers to Apple releasing new Ivy Bridge-powered systems.