For us the following problem is solved now! It might help other people with the same problems.
At our institute we have two new MacBook Airs 5.2 (2012) running Windows 7 64-bit Enterprise under Boot Camp.
They both cannot used for Windows-presentations with VGA beamers by using the mini-DP-to-VGA adapter (with Mac OS X 10.7 it just works fine). First of all, the suggestions from Apple phone support were not of any help at all (try PRAM reset, try SMC reset, ...). We tried an external USB-3.0-graphics adapter made by SUNIX (it failed, probably since Apple blocks other graphic cards in the EFI).
Then we saw the following article, which describes the same problems with a Dell XPS, but also using the Apple mini-DP-to-VGA adapter: