Apple's Magic mouse scroll button is not working on windows-7
Following your initial installation of Windows 7 via Bootcamp you need to go a step further, boot into Windows and install the Bootcamp drivers supplied by Apple.  These can be found on your Mac's installation media or by searching for "Boot Camp Support Software" at Apple's support downloads page.  The installation of this software enables functions such as your Eject key, iSight camera, etc, as well as functionality for your Magic Mouse.

Hope that helps!