Many dojo simply have a shomen; some dojo have a kamiza.
Walking between an instructor and a kamiza is a huge faux-pas.
The pattern remains present in dojo that have a shomen that is explictly not a kamiza.
For a fuller explanation of the way space is organized in a traditional dojo and why this might be so, Dave Lowry's article What Puts The Tao in the Dojo
is a good start.
I need to read that article carefully when I have more time.
I would never, say, enter the mat between the instructor and the kamiza / shomen. But often he will spread us out in groups down the mat to practice a technique. He might walk down the mat, away from the shomen, to watch the groups in back. The pairs in front would inevetiablely be between him and the shomen. Is that OK since they weren't the ones to move there?