I think it's possible that Steve Gibson is right on this week's Security Now podcast. He suggests that perhaps the NSA has tapped into the ISP just upstream of the 9 companies listed. Then, they copy all of the data coming into and out of the fiber going to the Google datacenters. This fits with the name Prism, and is corroborated by the 2006 revelation about the NSA-controlled room in the ATT building in San Francisco.

Edit: this does rely on the NSA having access to routers, not servers, so it still isn't exactly what they said