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Better alternatives would be to have your onclick handler first check to see if the value of the text box is "Enter Search Term Here" before wiping it out. However, this approach (as well as the original suggestion) has an accessibility issue. For accessibility reasons, the label for the text box should preceed the text box, so that someone who is blind or using a screen reader knows what the text box is for before getting to it. One method that I read about at A List Apart called "Making Compact Forms More Accessible" does what you're looking for and still remains accessible. (unfortunately, I can not post the link here).
[edited by: Fotiman at 4:57 pm (utc) on Aug. 2, 2007]