Well, I feel it is important to put together a toolkit of controls to use when developing for a client.  One of the controls I have created is the Textbox Filter Extender which will allow you to limit a textbox to numeric, alpha, or alphanumeric characters.  I've post the control for anyone to have access to and use.  Free free to check it out over in the development tools section.   If you have any questions, comments, or suggestions for improvements please post them to the proper forum section.