On my fully rigid SS, I run a 26x2.5 up front, and a 26x2.2 in the back. The treads are different as stated above for getting the most out of that wheel. Another reason I run a bigger tire up front is to help soak up the bumps. When I first built up the bike I ran a 2.2 upfront. Since putting a 2.5 up front, I've noticed some difference in the steering characteristics. But I'm racking that up to a tire with better side grip.
Originally Posted by cyccommute
The front and rear of the bike do very different things. The rear tire needs to have grip for traction as you climb while the front needs traction for steering. The traction for the rear needs to be more in the center of the tire rather than on the edge while you want good edge control for a front tire.
A good example of what cyccomute is saying, would be the Panaracer Smoke and Dart tires. The rear (Smoke) has a "paddle" like design for laying down grip for acceleration/climbing. While the front (Dart) has an arrow like pattern for more lateral grip.
http://www.panaracer.com/eng/products/mtb/xc.html