While I think the suggestions for what to wear are great, the most appropriate guideline that has been given is to ask what to wear. As has been said, "casual" can cover a wide variety of clothing. If everyone else is in tank tops and sandals, even a nice dress shirt would be over-dressed. On the other hand, if everyone else is in dress slacks and shirts, then those sandals would be out of place.

We can all argue over what "should" be worn based on the established conventions, but the real authority in any case is the host or hostess.