Larry's (Instar) the man when it comes to diseases. Richard is also a very good source of info. Richard fights disease on a regular basis because he's constantly getting in new, stressed fish. He's also one of the few LFS owners that quarantine fish. Drop him a PM, he's very helpful. He may not catch this thread, but he's good about answering PM's.

My opinion is that ich is usually the result of some form of stress in the tank. Most Tang's are carriers of ich but it doesn't get to the full blown stage until some sort of environmental, nutritional or behaviorial source of stress occurs.
I'd take a look at what is going on in your tank. What has changed?

Gary