A while back, I wrote about creating a Please Interrupt Me sign for the reference desk. We've had great feedback. Many people have commented on it, and we've heard "I like your sign" quite a few times, including from the President of the University!
It hasn't been without its drawbacks, however. Occasional abuse of the sign does happen, particularly during summer day camp. Young boys between the ages of 8 and 12 seem to think the sign is an open invitation to annoy the crap out of the librarian. Oh well, you'll have that. We can also choose to take down the sign temporarily if the kids become too obnoxious.
Overall, however, the benefits far outweigh the few drawbacks.