We can't completely assume Barr is pulling away all mccain supporters, as I can't imagine someone that could vote for Barr, OR Mccain. I'm sure there is a dent though.
All Zogby:
http://www.zogby.com/50state/
Ohio
Obama - 43%
McCain - 38%
Barr - 7%
Virginia
Obama - 44%
McCain - 39%
Barr - 5%
Arizona
Obama - 42%
McCain - 39%
Barr - 7%
Nevada
Obama - 38%
McCain - 38%
Barr - 9%
South Carolina
Obama - 42%
McCain - 41%
Barr - 6%
You get the point.
All Zogby:
http://www.zogby.com/50state/
Ohio
Obama - 43%
McCain - 38%
Barr - 7%
Virginia
Obama - 44%
McCain - 39%
Barr - 5%
Arizona
Obama - 42%
McCain - 39%
Barr - 7%
Nevada
Obama - 38%
McCain - 38%
Barr - 9%
South Carolina
Obama - 42%
McCain - 41%
Barr - 6%
You get the point.