I have a new demo at http://munich.schwarz-interactive.de/autocomplete.aspx [1] which will use a textbox and Ajax.NET Professional [2] 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.