We can't bind thing properties directly to a dropdown list in a mashup. Mashups will generally only fetch data from services.
However, there is a built in service all Things have called GetProperties. Adding this service to a mashup should allow you to essentially bind your custom properties to the mashup.
This built-in service may not be enough for your use case of creating a drop-down list (unless you already have an infotable as a property you'll work against). So generally you will need to write your own service that returns the properties you'll need to bind to your list widget.
