Absolutely toss the lettuce. It's not worth taking a chance of your chickens getting infected. E. coli is one bacterium that we can get from chickens and they can get from us. I believe I've run into a situation here on BYC where the chickens got sick from eating unwashed contaminated produce.
Simply washing isn't going to remove the bacteria if it was grown in untreated sewage sludge which happens more than we are aware.
I agree with tossing it as a precaution (though if you've been eating it and haven't gotten sick, probably a good chance it's not contaminated).
I've been growing my own lettuce the last 2 years, as well as a number of other greens. No worry about recalls that way! Greens in general, especially the bagged/pre-processed type, seem to come up a lot as far as recalls go.
if it's any consolation, I'd presume chickens digging and eating in dirt etc, would have better immune system and would have resistance to small quantities of e-coli that could be far more dangerous for humans.
Some people collect scraps from restaurants to feed to chickens - who knows what kind of lettuces get in those and how any food scraps are being handled after they are no longer human food - chickens still manage.
PS: I am not subscriber to a theory of "you have to remove not eaten food scraps from the run or they can make your birds thick" either.
Toss it, and hope that there hasn't been any prior damage to your flock in feeding them it for the past 2 week's. There are entirely too many recall's going on with our food lately! What in the heck is wrong with Utah by the way? Ground beef and now the poultry? I get the notices from the FDA site.