The difference in budo is in unlearning one's inhibitions about physical threats and space. You have to become as absorbed in the immediacy and the process of being attacked as a painter becomes in painting a detailed landscape for hours on end. That is not so easy -- because it involves completely inverting one's sense of threat, about which some people have very deep inhibitions that they are often unable to remove without grave difficulty.
Erick, I'm not sure if I can get behind that totally. Fear is a slightly different issue. When I was a bike messenger, I was under a near constant threat of personal injury. That experience taught me a few lessons about fear.
First, simply being exposed to risk hardens one to fear. After a while, you simply stop having emotional reactions to dangerous situations. You kinda grow apathetic.
But more significantly, as one grows in skill, you also gain a greater awareness of one's limits. With this sensitivity, you can push yourself more and more. One's limits are often much greater than a beginner thinks. A beginner may see an expert, and think they're doing really risky stuff. But the expert knows it's nothing they can't handle.
And this leads to a greater awareness of what IS and ISN'T actually dangerous. An expert boxer can bob and weave between punches with a cool detachment---not because he's hardened to fear, per se, but because he recognizes that his opponent is kinda slow (or whatever). He's never actually in real danger, so he feels no fear.
Anyway, I could continue to rant, but that's not the point here. My point is that Mushin and managing fear, in and of themselves, are seperate skills. In "standard" Aikido practice, without sparring, I think it's possible to achieve Mushin without ever really learning to deal with fear, since practitioners are never really in actual danger. Likewise, I think you can learn to manage fear without ever really entering into a state of Mushin.
PS---I wanted to add to my post above, about meditative Mushin. Zen monks attempt to carry that state into their daily lives, so that type of Mushin is potentially permanent. The others I mention are more situational.