I would take the tire off, cut it in half. Look to see if the cut is down into the cords. If not, glue the bitch back together and ride like the wind.

If it isn't that deep, into the cord or close to it, don't worry.

 

If it's not to the depth of wear markers, I'd say it's OK, just watch for tears in the rubber. Patch glue may be a good idea in this case, especially considering that it's near the center of the contact patch.

If you're really worried, have a patch placed inside the tire.