My girlfriend was looking into it for a while for her 15" PB. Her family is irish and she wanted to get a celtic knot pattern all around the apple symbol. She found a picture on the web that was really cool and the guys who do this were very helpful and worked with her for a while, but in the end she didn't do it. She couldn't justify the cost, for the pattern she picked it would have cost over $200 dollars. The cost all depends on the intricacy and size of the pattern you want etched. She might end up doing it some day, but as we are both in college and don't have a ton of money, it wasn't worth it at the moment.
And just to clear things up for some people, it is not actually printed on the PB case, they take a thousands of an inch of aluminum off the casing so there is no worry of the pattern "rubbing off".