Consumer Reports. org has published an article on line about salad greens in plastic clamshells or bags. The packages state "prewashed" or "triple-washed" and imply that you can use them directly out of the bags.
Well I admit, that's exactly what I do. For me that's the whole point, open and serve. Consumer Reports recently ran some tests and did find bacteria present but not E. coli O157:H7, listeria or salmonella- the three most serious threats for food-borne illness.
Several years ago I heard a representative of the FDA speak and he said that these products are "safe" but when questioned he said he did wash these products before using.
Consumer Reports recommends that you purchases these packages as far from the use-by-date as possible and rinse well.
This is one more reason to lobby Congress on the food safety standard of the U.S.