Bertrand Le Roy has moved his old client-side alert code over to CodePlex. You can learn more about it and check out the download here: http://weblogs.asp.net/bleroy/archive/2005/12/01/432016.aspx .
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Read the complete post at http://weblogs.asp.net/jgaylord/archive/2007/09/04/client-side-alerts-in-asp-net.aspx