I have used this myself and found it to be a great method for displaying my art heads. I used .css to swop my h1 heads with art. The visitors see the .gif art head and the engines see the h1 tagged text. I didn't do this as a black hat appoach, but to cosmetically display my heads well without loosing the value of h1 tags for search engines.

Yes, I think if it is used correctly is it very ethical.

Wayne Goodman, Website Design, Search Engine Marketing

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Smith O'Keefe and Associates, Marketing, Advertising, Internet and Multi-media