I do have a tendency to do it, more so if I'm really tired. We did two 1.5hr sets with a short break in between on new years eve, and I did zone out several times! Of course, if I'm playing something I know I really need to concentrate and be on the ball with, I'm fully focused, but if it's songs that I know inside out I can quite easily drift away.....
I think just the right level of zoning out really optimises my playing, but if I zone out too much I can lose track of what I'm doing!