If I were to use Windows 7 or 8 either via Boot Camp or Virtualisation Software on my Mac would my Magic Trackpad still function?
There is a table showing the features of the Magic Trackpad supported for Windows 7, Windows Vista and Windows XP on the About Apple Magic Trackpad (and 27-inch LED Cinema Display) Update for Windows on the Apple Support site.

http://support.apple.com/kb/ht4273

Windows 8 is not currently supported by BOOTCAMP. Boot Camp 4.0, OS X Lion and Mountain Lion: Frequently asked questions refers:

http://support.apple.com/kb/HT4818#3.1