One of the major poskim of our generation permits davening while wearing a Purim costume. He says that as long as you are serious it doesn't matter what you are wearing.
I find this psak quite surprising. Would one go in front of a king dressed like a clown?? So how can one daven to Hashem that way? Many of us are so careful to daven with a hat and jacket and suddenly a bright red clown hat and multi colored jacket. Or no hat and jacket because it doesn't go with his costume - dressed up like those friendly hicks who used to be the "fix it men" in camp. You know - the "dungaree-jeans-t-shirt-portruding-belly-holding-tool-box-cigarette-on-ear" look. If there is a hat - it is a baseball cap, preferably backwards.
On the other hand - it is a great limmud zchus for all of those Jews who daven that way.
