Oddly enough, I'm seeing somewhat the opposite problem on a MacBook Pro with the Bootcamp 2.1 drivers.
In XP, the brightness is always 100%. I can use the keyboard to change it to less than 100%, but then a second later it returns to 100%. I didn't have this problem with 2.0.
Can you change bightness with the Brightness tab in the bootcamp control panel?
"After rebooting in Bootcamp Windows XP, I installed the Bootcamp Update 2.1. After a Restart, back in Windows XP, I had a Screen Resolution of 640x480 Pixel an 16 Colors. And whatever I tried, it was impossible to change that."
Ahh... I figured that since the brightness was controllable over USB, that bootcamp was changing the brightness of the display, especially since you mentioned that reinstalling BC 2.0 brought it back to normal.
Do you think it is a brightness issue, or a driver issue? Do the brightness controls on the side of the display work? Can you adjust it with the nVidia control panel, it has all kinds of brightness and gamma features.
OK... how about this:
Is your cinema display USB cable connected? If it is, try unplugging the USB cable. Boot into Mac, set brightness to full (may be an unnecessary step) pull the USB cable, then boot into PC... it shouldn't be able to dim it unless it is talking to it via USB. At least that is my guess. I use an older cinema display on a PC, and it knows nothing about the backlight... and therefore doesn't change the brightness.
I've got a 2007 8 x 3.0GHz Mac Pro with a 23" Apple Cinema Display going via an Apple ADC->DVI adaptor.
With the factory fitted ATI 1900XT and Boot Camp 2.0 / XP SP2, everything was fine (except the ATI graphics card either overheating or attempting levitation every once in a while, but that's par for the course).
Upgrading to Boot Camp 2.1 (without doing XP SP3), and suddenly I'm getting the same dim monitor as the original poster. Installing "real" ATI drivers and the ATI CCC tool didn't do anything (there were brightness controls in the drivers, but they made the picture awful, unlike the keyboard-based brightness mac users are used to). Installing XP SP 3 made no difference.
So I re-installed everything (in terms of Boot Camp / XP SP2). Back in Boot Camp 2.0, it's rosy again. Install Boot Camp 2.1 (without any Windows updates at all over and above XP SP2) and the problem is back.
Re-installed all the Boot Camp / Windows stuff (each time reformatting by the way), this time having swapped the ATI 1900XT for an Apple Nvidia 8800GT. Although the mac no longer attempts vertical take off, and although I can now hear a pin drop, the picture issue remains: Boot Camp 2.1 dims the monitor, and nothing (in XP) can fix it. So it's not a specific graphics card issue, as it's happening under ATI and Nvidia cards.
As the original poster said, you can go back into Mac OS X, use the keyboard (or brightness settings in the Displays System Preference), and you're fine in Mac OS X. But go back into XP and it goes dim again. Interestingly it's not immediately dim in XP, it happens when the boot camp system tray icon (those things down by the clock in XP) loads... up until that point the monitor is fully bright in XP, assuming you'd just previously fixed it in Mac OS X prior to booting.
The solution for me was along the lines of what Moonlight Mac was suggesting: go into Mac OS X with everything plugged in normally, chuck the brightness up to your preferred amount, then remove the USB cable belonging to the DVI->ADC adaptor (which for me was going straight into the back of the mac). This prevents the mac pro (or you) from being able to adjust the monitor's brightness. Now boot into XP and you can actually see the desktop.
Aargh it's horrible, turn the lights back off!
Although it provides a makeshift fix, the disadvantages are:
 You still can't adjust the brightness in XP, you have to go Mac OS X -> plug cable in -> adjust brightness -> remove cable -> reboot into XP. For me I want it at full brightness all the time, so this limit doesn't bother me.
 You're a USB socket down on the deal (the ADC monitor & ADC->DVI adaptor give you 2 sockets and take up 1 (in the mac), whereas now you only have that 1 (in your mac).
So it looks like an issue between Boot Camp 2.1, and the ADC->DVI adaptor (or the Apple Cinema Display). The original poster suggested re-installing the Boot Camp 2.0 drivers, but I didn't attempt this so can't say if rewinding to Boot Camp 2.0 (rather than reinstalling, which was the path I took) works in my config. The reason I didn't want to stay on Boot Camp 2.0 was because 2.1 made the 802.11n Airport network actually usable under XP (fine in Mac OS X of course), and that was more important to me than the 1x USB socket that I have to sacrifice using my fix.
Hope this helps,
p.s. very glad to find this post as Google & Apple Support searches hadn't been turning up anything. I thought I was going mad
OK, this is a TOTAL shot in the dark, but what happens if you open the Device Manager, then from the view menu select "show hidden devices"
Under the System devices category, try disabling "Apple panel backlight"
After that, reboot into osx and make sure display is operating normally, then boot into windows and see if it stays bright (with the USB pigtail from the DVI to ADC adapter connected the whole time.)
If that doesn't work, there has to be a device or service that can be disabled to prevent the backlight from getting dimmed.
I was having the same issue as yourself, but with Windows XP.
I think I've found a solution. WinACD is a Sourceforge project to add Apple Cinema Display HID driver and control panel for Windows XP.
I installed it and restarted. It added back the activity to the brightness control button and added brightness control to the display settings. If you go to "Advanced" under the display settings, there will be a tab for "Apple Controls." Using that tab, you can set the brightness for the monitor, plus add keyboard controls for the brightness setting.
You can download it here http://sourceforge.net/projects/winacd/
Hope this helps.