There is a way to setup a unique number. This can be achieved using the special value of 'Instance number'. One thing to remember before I explain how to do this, the Instance number does not get created until after you submit your form. So as your users are launching new forms, the place holder for your unique number will not show the correct unique value until after the form has been submitted once. If this form is going to route to someone else to perform an action on it, they will see the unique number as they open the form.
The reason we wait for a form submission with the instance number, is in the case more than 1 person opens the form at the same time. If we created the instance number prior to form submission, that would mean that for every form opened, there would be an instance number. What if 1 out of 3 of those people submit the form, then you're left with holes in your numbering scheme.
In this example, lets say your unique number is '2009-00x' where x is the Instance number, sequencial from 1 - infinity. In your form layout you would drag a 'text input' box onto your form. You can choose a trigger to write in this field. In the associated behavior, use a 'set field', choose formula. in the first line, write '2009-00' in the text field. Now add a line, then choose the special value 'Instance Number'. Your all set.
