I am not a Catholic, and I do not believe the pope to be infallible. But I do believe that I can explain the Catholic doctrine on this issue.
The pope is considered to be infallible only when he teaches ex cathedra. (meaning "from the chair"). There are a number of conditions that must be fulfilled in order for this to be the case (the teaching must be public; the teaching must apply to doctrine; the pope must intend for the teaching to carry apolistic authority; the pope must intend to bind the whole church by his teaching). Because of all the conditions, "infallible" teaching is very rare. And because of the conditions, infallible teaching can only be done by a current pope.
So to be clear, infallibility does not mean that a pope is without sin. It also doesn't mean that a pope can't contradict another pope or make a mistake. Pope's are free to sin or err all they want without violating the doctrine of papal infallibility. The only thing that can't be done is to make a mistake while teaching ex cathedra. This is because when the pope speaks ex cathedra, he is thought to speak with the authority of the Catholic Church itself, which (from a Catholic perspective) is the authority of Christ himself. This is what supposedly makes the teaching infallible. So without the ability to teach with that authority, there is no chance to teach infallibly.
Short version: According to Catholic dogma, Benedict can no longer teach ex cahtedra, meaning he can no longer issue infallible statements.