Apple’s MacBook Air EFI Firmware Update 2.9 (for Mid 2011 models) was pulled on July 22nd because many users were reporting the update would fail to install, often times requiring the SMC to be reset afterwards to get the MacBook Air to boot again.

Apply re-released this update as MacBook Air EFI Firmware Update 2.9.1 on July 20th, and this problem has been resolved.