The ! prevents YUI compressor from removing the comment when it compresses. (It just removes 1 ! instead. Multiple !'s mean you can compress multiple times without loss of the comment.) It's just an extension, but not part of javascript itself.
Documentation is here. Search for 'C-style comments'.
also, I'm not aware of any other compressors that respect the !. Packer, closure compiler, shrinksafe, and jsmin do not respect it at least.
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