Just did some half-assed research... Do what's best for you and the safety of those around you. Before I found this, I remembered the Lincoln penny tip and measured. It looked to be 3/32nds to me (as in this article).

http://trucks.about.com/cs/tires/ht/...tm?terms=tires

A quote from the article here...

3. Look in the grooves between tire tread for raised patches of rubber called wear bars. These 2/32-inch tall patches will help you identify a worn out tire. (In most states 3/32-inch is the mimimun legal tread depth.)

4. If tread is worn to a level where wear bars are flush with the tread it indicates that tread depth is 2/32-inch or less. Replace worn tires.

5. If your tires do not have wear bars, place a US penny into a groove between tread, with Lincoln's head down. If tread is at or beyond the top of Lincoln's head you have at at least 3/32-inch of tread left, a satisfactory level.