Survey USA polled head-to-head match-ups in Pennsylvania and New Mexico, asking voters whether they preferred Obama or McCain, each with one of four running mates: John Edwards, Kathleen Sebelius, Ed Rendell, and Chuck Hagel for Obama, and Mike Huckabee, Mitt Romney, Tim Pawlenty, and Joe Lieberman for McCain - a reasonable mix of former primary opponents, swing state governors, and opposing party mavericks.
The results show Obama/Edwards and McCain/Huckabee being the strongest tickets. Obama/Rendell does well in PA (no surprise), and all of the McCain tickets do relatively well in New Mexico.
Ideally, Survey USA would run polls like this in every state, which would give more data toward the question of whether vice presidential nominees matter. These two polls seem to suggest that they do.