There are primaries coming next week in 3 Senate races, and the incumbent stands to lose in several of them (possibly all 3!). The one people are really freaking out about, though, is Joe Sestak's sudden and consistent lead over Arlen Specter in PA.
Speaking specifically of this race, I'm surprised anyone else is surprised. Specter is a truly loathsome senator, the Platonic form of the gutless, "finger in the wind" politician whose only principle is being ardently pro-reelection. He's been a Democrat for all of a year, was cowed over and over again for 8 years by George W. Bush (when he wasn't ardently supporting him!), and the way he's stepped into the party and demanded that they give him all their support just because he's an incumbent has been embarrassing.
More broadly speaking, I'm also surprised that people continue to put so much stock in election polls so far out. Seemingly static election numbers regularly go wild 2 weeks from election day because people don't start paying attention until then. This is especially true with primaries for whatever reason. Remember, for instance, that neither Barack Obama nor John McCain had a prayer of winning a month before the Iowa Caucus. In '04, Howard Dean was cruising to a blowout win just a week and half out. You just don't know until a few days before Election Day.