
When slanted down, the mechanical button is at the front, activated by the fingertips instead of the thumb. If you don't like this, try turning on tap-to-click and 3-finger drag in the trackpad control panel. (You may then want the defaults write that moves navigation swipes from three to four fingers, displacing Exposé and application switch).
Type this in terminal:
defaults write com.apple.trackpad.orientation TrackpadOrientationMode 1
The orientation detection should become active after rebooting, reconnecting the trackpad, or changing other settings in the trackpad control panel. Just rest 5 fingers on the Magic Trackpad after turning it around and the cursor will thereafter move in the desired direction.
[crarko adds: I haven't tested this one. It has been suggested that the following command will work where the other does not. Give it a try and let us know. You will need to supply your admin password for this.]
sudo defaults write com.apple.MultitouchSupport ForceAutoOrientation YES