This issue appears to be resolved (well, it never required resolution, just the proper drivers) by installing only part of the Apple Bluetooth Update 1.0 for Windows.
The update can be found here:
http://support.apple.com/kb/DL952
Once the package is downloaded, you must extract the files using WinRAR or any other similar ZIP/RAR utility.
You
must extract the files from the update. If you simply try to install the entire update, you'll receive a prompt stating the update isn't required for your specific model, as this update is intended for the iMac.
Once extracted, open the target directory and double-click the AppleWirelessMouse.exe that applies to your Windows OS (32 or 64 bit).
I'm using my Magic Mouse with Boot Camp and Fusion 3.0 running Win 7 and both vertical and horz scrolling work just fine. The Magic Mouse also feels a bit more responsive. Both are welcome improvements.
I'm not quite sure why Apple hasn't provided more information on this, it seems they're okay with the implication that Magic Mouse only works in Mac OS X.