I agree with Steve's definition. I think one aspect of the Zone is that you are making progress on a problem, which is almost like bio-feedback and gains momentum. I think that's why it is so satisfying when you "come out of the Zone" and realize how much progress you've made.

Working in cubicles does make it hard to get/stay in the Zone. I'll put on headphones and listen to music - in my case, classical/instrumental to get back into the Zone, but once I'm there, I don't need white noise. Also, I am definitely a morning person. I used to get up at 4:15 am, and get 2 solid hours of Zone before I got the kids up, off to school and left for the office! (Now, I get up at 5:30 and out the door to beat the morning rush hour, and get settled in my cubicle before other people start to show up.)