Last week, we had the author of TDD for a Shopping Website LiveProject. Friday at 11am Ranch time, Steven Solomon will be hosting a live TDD session just for us. See for the agenda and registration link
I'm not sure how you would cast a number to a string through an EJB QL. However, you could always implement the same thing as a home method, and use an ejbSelect that does the same thing that returns the number. Afterwards, just have your home method cast it to a string. Just my 2 cents.