Apple released EFI Firmware Update 1.8, a firmware update for Mid 2010 iMacs that enables Lion Internet Recovery for the first time. Lion Internet Recovery allows Macs to recover or reinstall Lion over the Internet.
Apple’s sparse patch notes for the firmware update:
About iMac EFI Firmware Update 1.8
This update enables Lion Recovery from an Internet connection on iMac (Mid 2010) models.
For more information about Lion Recovery, please visit this website: http://www.apple.com/macosx/recovery/.
As of this writing, the firmware update isn’t showing up on Apple’s Downloads site, but if you have a Mid 2010 iMac, you should find it in Software Update as a 2.9MB download.