Recently we added a custom field at the request of a user. We labeled it identifier, and it is located below the priority field in the editor. The label for this field is customizable via the settings menu. This is why I love this model. In my day job, I NEVER talk to customers.
Here' a shot of the editor and the new custom field.
This field is indexed and you can filter on it from the task list.
Todo Q is available on the Android Market.