Apple has released an update for late 2012 iMac models that fixes issues related to sleep and Thunderbolt, as well as compatibility when using 5GHz Wi-Fi frequencies. The update is recommended for all "late 2012" iMacs.
iMac EFI Update 2.0 can be downloaded from Apple's support site or through the Mac App Store on affected iMacs.
About iMac EFI Update 2.0
This update is recommended for all iMac (late 2012) models. This update contains general performance fixes pertaining to sleep and Thunderbolt, and improves compatibility when using the 5GHz band in Wi-Fi.
Boot ROM or SMC Version Information: After Update is complete:
Build version: 010A.05
ROM version: IM131.88Z.010A.B05.1211151146
It's not clear if this is in addition to the Wi-Fi Update released earlier today, or if it includes that fix as well as a few more iMac-specific ones.
Update: Apple has released a similar MacBook Pro Retina EFI Update v1.1 for the 13-inch Retina MacBook Pro.