A caveat: This is a poll of registered voters, not likely voters. For Perriello to stay in a solid position, he'll need to overcome the general trend over the last year of Democratic turnout having been less energetic than the Republican vote. (This was especially pronounced in last year's gubernatorial race in Virginia.) By a coincidence, the registered voter pool said they voted 51% for John McCain in 2008, and 48% for Barack Obama, matching the actual district results.
Perriello's district voted 51%-48% for John McCain in 2008, while Perriello himself won a surprising upset victory over incumbent Republican Rep. Virgil Goode, by a margin of 50.1%-49.9%. Since then, he's been a top target of the GOP, and has attracted a large field of GOP candidates against him.
Perriello has generally focused on a platform of economic populism, while also balancing it out with some socially conservative stances. He voted for the health care bill, for example, but he also voted for the Stupak Amendment to restrict insurance coverage of abortion.
As an example of the general impression that Perriello is highly vulnerable, Stuart Rothenberg wrote in December: "Freshman Rep. Tom Perriello (D) seems more interested in doing what he thinks is right than getting re-elected. That's the only way to explain his votes supporting House Democrats' cap-and-trade and health care reform bills. State Sen. Robert Hurt (R) is expected to challenge Perriello, and the Congressman is in deep, deep trouble."
This poll, the first real data point we have for the election itself, shows that the overall picture is much less clear. Perriello is of course a Democrat in a very tight district, and is in for a tough race this year, as his approval rating is under 50%. But he is by no means a goner, and for the moment is ahead of the competition.