The Magic Trackpad is manufactured to be used in the way you see in the photograph that heads this post, but some users may find it much easier to use it inverted, a possibility that Apple does not offer initially.
As I read in Mac OS X Hints It can be activated by putting this command in Terminal:
defaults write com.apple.trackpad.orientation TrackpadOrientationMode 1
To return to normal operation it is recommended to delete the file created with the command, and I do not assure you 100% that it works because I do not have a Magic Trackpad, but it would be strange if it did not.