Which way do I orient my bike tire's tread direction? and why? I have a mountain bike tire that has stamped on the side something like: 

&lt;----------front   rear------------->

Which seems like a pretty clear indication that the tire should be oriented in one direction when used in the front, and the opposite when used in the rear.  Is that right?  This is counter intuitive to me - I would have expected tire tread to face the same direction on both wheels - like cars.  

If so, is this true in all cases?  For example, road bike tires with tread to handle rain?

And finally...why?
The tires have more traction in some directions than in others.

Your front wheel does most of the braking; your rear wheel does all of the propulsion. 

Tires are oriented to give the most traction to the primary fore-and-aft forces.