Not Virginia creeper, which has five leaves. Or poison ivy. I think. I'm trying to figure this one out. We found in the Meadow on a live oak.
UPDATE May 29, 2010–Case solved. Sorrelvine, cow-itch vine (Cissus trifoliata), a member of the grape family. According to Wildflowers Trees, and Shrubs of Texas, it can cause contact dermatitis in some individuals.