There seem to be at least two major VP possibilities - Evan Bayh and Tim Kaine - who potentially stand to help Obama carry a swing state.  Mitt Romney may help McCain win Michigan or, less likely, Tim Pawlenty may help McCain win Minnesota.  

However, I keep hearing people say, "The Vice Presidential nominee never actually matters in terms of carrying a state."  These people go on that the last time a VP nominee truly carried his home state was in 1960, when LBJ carried Texas.  That got me thinking, how often has a VP nominee really even been asked to carry a swing state since then, and how often has that VP nominee succeeded?  So here goes - here are the VP's selected by presidential nominees since Kennedy:

1964

Hubert Humphrey (D-MN) - Johnson won MN easily in landslide election.

William Miller (R-NY) - Johnson won NY easily in landslide election.

1968

Edmund Muskie (D-ME) - Humphrey won ME easily.  Muskie may have helped put Maine out of reach.

Spiro Agnew (R-MD) - Nixon may have wanted to make a play for MD, but he did not come particularly close to doing so.  

1972

Sargent Shriver (D-MD) - Nixon won MD easily in a landslide election.

1976

Walter Mondale (D-MN) - Mondale probably helped to put MN out of reach, but Carter very probably would have won it anyway.

Bob Dole (R-KS) - Ford won KS by 7.5%.  Dole probably helped, but it was not really in play.

1980

George Bush (R-TX) - TX was probably a place that Reagan wanted to shore up, given that Carter won it in 1976.  In the end, Reagan won TX easily.  

1984

Geraldine Ferraro (D-NY) - Certainly one that Mondale would have hoped to win.  Reagan won narrowly as part of his landslide.  

1988

Lloyd Bentsen (D-TX) - TX not in play.  Safe R.

Dan Quayle (R-IN) - IN not in play.  Safe R.

1992

Al Gore (D-TN) - Clinton won Tennessee by nearly 5%.  Gore almost certainly helped, but Clinton ran strongly in the region and may have won without Gore.

1996

Jack Kemp (R-NY) - NY not in play.  Safe D.

2000

Joe Lieberman (D-CT) - CT not in play.  Safe D.

Dick Cheney (R-TX/WY) - TX/WY not in play.  Safe R.

2004

John Edwards (D-NC) - NC not really in play.  Safe R.

Overall, these VP nominees were 9-7 in their home states.  However, none of them were really asked to carry "swing" states.  The one exception may have been Gore in 1992, but his home state was one where fellow Southerner Bill Clinton would probably have run strongly without Gore, and geography was probably not the main motivation in selecting Gore.  Then Gore went and lost TN in his bid for Preznit in 2000!

A few other home states (Dole 1976; Mondale 1976; Muskie 1968; Agnew 1968) may have been thought to be in play at some point, but were not really "swing" states and did not end up close.  The rest (Edwards 2004; Lieberman 2000; Kemp 1996; Quayle 1988; Bentsen 1988; Ferraro 1984; Bush 1980; Shriver 1972; Humphrey 1964; Miller 1964) were either safe states or were part of landslide elections where no seat was safe.  

My conclusion, then, is that the ability of a VP nominee to carry a swing state is largely untested since 1960 because VP nominees have by and large not been selected to carry swing states.  The closest example - Al Gore in 1992 - came up roses.  

In short, there does not seem to be any evidence in recent history that a VP nominee generally will not be able to carry a swing state if selected for that purpose.  It's just untested.

So take that, people who throw this theory around, including my colleagues with whom I had lunch today!  HIYAA!