Yes, butter contains trace amounts of casein. So if one is on a strict casein-free diet, as some children with autism, ADD, and ADHD are, then butter would not be acceptable. Ghee, which is clarified butter, is casein-free. [reference]
The amount of casein that butter contain is not a large amount, however. Casein is a protein found in dairy. And if you look at the box label, it typically says 0 grams of protein.
Butter has less casein than milk. Depending on a person sensitivity, they may be able to tolerate butter even though they may not be able to tolerate milk.