I just ordered one, thought I'd post in case anyone else was interested. The Audiovox XM Unit including FM Modulator is free when you buy three months service for $38.
Go to http://partners.XMradio.com enter code OPIANTXM6 scroll down, accept the terms and conditions and click "Shop Now". There are a variety of discounted XM Units, the free Audivox is on the second page.
No affiliation or benefits to me from this, unless you insis of course...!