Even though the new iMac was just released on Tuesday, there's already an system update available specifically for the new machine.
The iMac (late 2013) EFI Update 2.1 is for iMac users who also use Boot Camp to run Windows 7 or 8. The updater fixes a problem where a black screen appears when you have both an external optical drive and a USB thumb drive connected to the iMac while installing Windows 7 or 8. The update also makes the iMac boot into OS X by default if you've installed Windows 8.
If you happen to have a brand new iMac, congrats! And always back up your data before installing the update.