My Macbook Air 13″ mid-2011 running Bootcamp with Windows 7, had an issues with sleep modes. Whenever I closed the lid or asked Windows to sleep, a reboot was necessary.
After a failed attempt at resetting the SMC, the solution was something else. A selected startup disk was required.
Image may be NSFW.
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This blogpost pointed me in the right direction:
http://osxdaily.com/2011/04/05/how-i-fixed-my-macbook-air-11-6-wake-from-sleep-problems/
I selected a startup disc in both MacOSX and the Bootcamp configuration and the Mac now supports sleep mode again.
