According to Donald Schaffner, an expert on microbial risk at Rutgers University, you actually increase the risk of cross-contamination when you wash pre-washed lettuces. That's because your kitchen—no matter how much of a clean freak you are—is like Club Med for bacteria. There are people and pets and sneezes and all kinds of foods sharing a relatively small space.
At the processing plants, that lettuce was washed in very cold chlorinated water or acetic acid (the main ingredient in vinegar), and it was handled in a highly controlled environment where it had little or no contact with other foods. So it's probably freer of potential contaminants before you open the bag than after.
And just think, you never have to feel guilty about not washing pre-washed lettuce again.