Great question. There is not just one answer to that one. Sure, sometimes it is the desk, but it certainly doesn't have to be.
In a living space I would say that the furniture all faces the focal point, such as the center of the dining table, or the fireplace or television. In a small office, there might be just one chair, which faces the computer monitor. ;-) A larger office which has guest chairs facing the desk would have the desk, or wall behind the desk, as the focal point. The question is, when you enter the room, what do you want the eye to go to?
Feng Shui would say that the chair should not be facing the wall, but that rather the desk should be in the center of the room, where the chair is in a position where you can see a guest enter the room, not have your back to them. This setup would again have the focal point be the desk, or the wall behind the desk.
Here is an example where the focal point is the chair:
In the seminar, I talked about breaking a pattern, and creating a focal point- This office below is a good example of that: Notice above how you have bookcase, break, bookcase. The break lines up with the cute zebra chair and the mirror, creating a central dominance.
Here again, the focal point is the wall behind the desk, created with an accent of wallpaper:
In this small space, the focal point is not the desk chair, or wall behind the desk, but rather I would say that the focal point is the art:
In the office below I would say the focal point is the rug:
What I find as an interior designer is that I do have an "ideal". I like the focal point to be the wall behind the desk, with a secondary focal point in the chair and the desk itself. (the photo with the beige zebra chair is a good example) I like the main office chair to face into the room, instead of face the wall. This is how I have designed my personal office space.
However... I frequently have clients with restrictions that prohibit this. My cliet will say, "we need two chairs, two computers, six file drawers, two bookcases, and a sofa sleeper. " The restrictions are too many to also be able to treat the office with the focal point that I would like to. Sometimes function has to come first, forcing me to design some built-ins rather than be able to put a pretty desk in the center of the room.
Which home office photo is your favorite?