Apple released a firmware update on Tuesday for new iMacs introduced earlier in the day. iMac (late 2013) EFI Update 2.1 focuses on Boot Camp related issues and Windows support. Boot Camp is Apple's boot technology that allows Macs to boot up into either Windows or OS X.
Apple's patch notes:
This update is recommended for iMac (late 2013) models. This update addresses an issue which may cause a black screen to appear when installing Windows 7 or Windows 8 using Boot Camp Assistant if both an external optical drive and USB thumb drive are connected to the system. This update also ensures that the system will boot by default into OS X after installation of Windows 8.
Apple just released the new iMacs Tuesday morning, but if you're a lucky duck who already has one, you can download the firmware update from Apple's site as an 8.7MB download. It may also appear under Updates in the Mac App Store.