Sheldon Brown has something to say about that too:
For off-road use in soft surfaces, there may be some merit in paying attention to the tread orientation, though this is far from certain.
Ideally, you would like the front tire to offer maximum traction in the braking direction, while the rear tire would normally be oriented to produce maximum traction for drive forces. Thus, if a particular tread pattern is perceived to have better traction in one direction than the other, it should be facing one way if used on the front wheel, and the opposite way if used on the rear wheel.
way2low: If you are new to biking, you really should bookmark, and take a peek at Sheldon's website.
http://sheldonbrown.com/ Easily the single most useful site on biking when it comes things mechanical.