It doesn't happen often, but when it does it an be a big pain. Here is a nice tutorial over at our fellow blogger - the Photo-Smith - on how to get rid of those naggy moire' pattens we sometimes see in men's patterned jackets, ties, satin dresses and the like.
He gives the tutorial a nice walk through with a thorough explanation. Mr. Smith has I nice site with some really good info - I recommend checking it out. Here is the link to Photoshop Moire removal tutorial.