Could ball lightening be a form of plasma?

Ball lightning could definitely be some atmospheric pressure plasma phenomenon. You can make a pretty impressive ball plasma by discharging a kilojoule-scale capacitor bank into a bucket of salt water. Check out Free-Floating Atmospheric Pressure Ball Plasma. In most of those pictures they're using a copper sulfate solution, but that's not essential (sodium chloride also works). These ones only last a (significant) fraction of a second, but I'm sure if you made a larger one (e.g. by a lightning strike), they could last longer.

BTW, this was the subject of a killer science fair project: http://www.youtube.com/watch?v=SE6sbaNsKoc