I have a new demo at http://munich.schwarz-interactive.de/autocomplete.aspx  which will use a textbox and Ajax.NET Professional  to search for customer names (1st textbox) and then for the orders of this selected customer (2nd textbox).
There is a big difference to other AutoComplet boxes. You can use any AjaxMethod to do your search. There is an example done with returning DataSets, default would be something like string (arrays). The second textbox is using the CustomerID from the first textbox, not the CustomerName. This is also very different to other controls. If you have a look in the source code you will find both controls and how they are rendering the lines.
I didn't test the control in all browsers, IE and Firefox should work.