I have a 22" external monitor connected to my MacBook Pro (late 2011) via Thunderbolt to VGA. When I boot into Windows 7 x64, the screen seems to work and is mirrored during the boot screens, then it goes blank at the login screen. It stays blank after I log in to Windows. The built-in screen on the laptop works fine.
The monitor works fine in OS X Lion.
Best Answer
Note that it appears the Thunderbolt may work a bit differently on Windows in Bootcamp than it does on the same computer when running OS X. As noted below on Thunderbolt ports and displays: Frequently asked questions (FAQ)
Additionally after trying the above, you may need to manually activate and configure the 2nd display in Windows 7, it may not automatically configure and turn on with the first connection attempt.
Make Sure you have the Apple Provided BootCamp Drivers installed.
Right click on the Windows 7 Desktop, click on "Screen resolution".
From there you may need to click on "Detect" and further configure the add on displays resolution and so on.
If the above does not work it appears that there may be some driver issues with the graphics card drivers in Bootcamp that may be causing issues with the Thunderbolt a.k.a. the Mini DisplayPort to VGA adapter. There are quite a few discussion occuring on Apple's Support forms and on the internet dealing with this issue. One solution found:
If all else fails I recommend contacting Apple and letting them know of the issue so that they can develop a fix or workaround for it.