One of the bad times with CustomValidators in ASP.NET 1.1 was that the validation event will not fire when the ValidationText property was empty (i.e.: a textbox was empty). This was a bad time, because it required you to add a RequiredFieldValidator to ensure that the field was completed. The CustomValidator now has a ValidateEmptyText property. Setting this property to true will force the validation event to fire even when the control that is getting validated is empty.
Read the complete post at http://www.dotnettipoftheday.com/blog.aspx?id=146