Definetely. I would 100% wait until the subscription runs out and the radio is deactivated. Right now, they're giving you six months of free Sirius. If you add it to your account now, you may end up paying for those six months. In six months, we'll be at the end of June, and we may have an idea of the contractual situation of several important players at Sirius at that point.
And if you have a plug and play radio, you can always use that for a few months in the truck while you wait until November or December of 2010.
Edit - I just saw your post in the other thread. Even though you listen to other stuff besides Howard and Bubba, if they leave, it may still put the company in an unfavorable light, which could affect your subscription decision.
Still..wait until your truck no longer receives a Sirius signal, then make a decision.