Send the W-9. You need to in order to obtain payment. That client is or was in the process of closing their books for 2006. If you don't send them a W-9, chances are you will be the one in trouble. They would notify the IRS when they file their own tax papers.
If I were you, get the W-9 ready and then call before you send. Say..."by the way, I was wondering if you noticed my account has been unpaid since November". Anyone sole preprietor or LLP who receives revenues over $600.00 per year, needs to produce a W-9.