A survey of milk and lettuce in 15 US states found perchlorate (a toxic chemical used in rocket fuel) at unsafe levels. The FDA are not making any recommendations yet, but studies found an average concentration of 10.95 parts per billion while the recommended limit is 6 parts.

High concentrations of perchlorate can disrupt the thyroid gland, which regulates metabolism and is linked to the development of motor skills in children.

It's not clear from the Wired article Rocket Fuel in Milk, Lettuce if lettuce specifically accumulates the toxin (as compared with other green vegetables), but I would certainly suggest that anyone who is pregnant or with very young children should look into this some more.