A symmetric air foil would be a good example of one that can fly upside down or right side up without a degradation of it's properties. For non-symmetric air foil, all it takes is a small adjustment of the ailerons to give it lift in the upside down direction.
If the plane's lift were all thrust generated, it would be pointing straight up! Angle of attack, ailerons etc. can all explain it however.
There's an old thread about this as well: http://www.physicsforums.com/showthread.php?t=130414