Jim - There is another discussion we had about this issue on the Forum here which you can read through:
http://www.photographythailand.com/Forum/index.php/topic,738.0.htmlI recently started doing it with all images I post on the internet because some of my work is now being sold through stock photo agencies and I have to be more careful about what I put online in any public domains. Otherwise, I probably wouldn't be bothered with it because it does disrupt the visual experience for the viewer as David mentioned. Also, some of my general thoughts on this issue are discussed in that previous Forum post as well.
Good question to raise though
