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 2.0It'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.
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
Update: Apple has released a similar MacBook Pro Retina EFI Update v1.1 for the 13-inch Retina MacBook Pro.