sswitze1,
I think I know why you can only control your brightness from 0-50%.
I made an interesting discovery on using Boot Camp with PCs. Obviously, when PC users install Boot Camp 3.0 + Boot Camp 3.1 Update + Apple Magic Trackpad and 27-inch LED Cinema Display Update, a lot of unnecessary drivers are installed. Believe it or not, it seems there is only one single standalone file needed to control the ACD27" brightness settings on the PC: AppleControlPanel.exe.
Each update (Boot Camp 3.0 + Boot Camp 3.1 Update + Apple Magic Trackpad and 27-inch LED Cinema Display Update) contains a unique version of AppleControlPanel.exe
Using UniExtract, I first extracted AppleControlPanel.exe from the Boot Camp 3.1 Update. When I ran it, the Boot Camp Control Panel came up, and sure enough when I tried to change the brightness I only had control from 0-50%!!!
I then extracted AppleControlPanel.exe from from the Apple Magic Trackpad and 27-inch LED Cinema Display Update, and running this version gave me brightness control from 0-100%.
For some reason, it appears the Apple Magic Trackpad and 27-inch LED Cinema Display Update is failing to update your AppleControlPanel.exe. What you need to do is uninstall Boot Camp from your PC. Then download the most current version of AppleControlPanel.exe which I have posted here:
http://rapidshare.com/files/424506361/AppleControlPanel.zip
Now when you run the exe (make sure Boot Camp Services is uninstalled first!!!), your Boot Camp Control Panel will load up and you should now be able to control brightness across the full range!