In the tradition of the Designer family of products, we made it extremely simple to add AJAX type-ahead to an input edit control. Simply check the box, and tell us what the choices should be and you're done. For more dynamic lists, compute the values on the fly - for instance, do a view lookup and show column values for the choices. Or, point to an external URL that contains the list.
Pretty cool - why bother coding it yourself when we've done all the heavy lifting!