After settling on new royalty rates, Pandora is compensating their increased costs by limiting users to 40 hours per month with a $0.99 monthly fee for the remainder of that month. From their blog:

The revised royalties are quite high – higher in fact than any other form of radio. As a consequence, we will have to make an adjustment that will affect about 10% of our users who are our heaviest listeners. Specifically, we are going to begin limiting listening to 40 hours per month on the free version of Pandora.

In any given month, a listener who hits this limit can then opt for unlimited listening for the remainder of that month for just $0.99. In essence, we’re asking our heaviest users to put a dollar (well, almost a dollar) in the tip jar in any month in which they listen over 40 hours. We hope this is relatively painless and affordable–the same price as a single song download. (Alternatively, they can upgrade to “Pandora One”, our premium version which offers unlimited monthly listening in addition to its other benefits).