If you stop paying, the songs will expire and you wont be able to listen to them.
Steve Jobs doesn't want Apple to have a subscription type service. He doesn't like the idea of wasting your entire investment by cancelling.
Tomorrow (Thursday 28th April 2005) signals the 2 year anniversary of the iTunes Music Store launch in the US. It is possible that a subscription service could launch (as well as several new countries including Australia, Sweden, Norway, Denmark and Switzerland). Although a monthly service seems unlikely.