I don't agree with some of what Ken says. The tree is significantly damaged in a way that rot is likely. Rot can significantly weaken the trunk and create a tree that is much more likely to fail. As the tree ages and grows, the rot is likely to spread and the tree may be more likely to collapse, especially in windy conditions. Whether the tree will be able to successfully encapsulate and block off the rot is an unknown, so a variety of outcomes is possible. It appears that the tree was planted incorrectly, and this has likely contributed to the problem your tree now faces. The exact, and complete, cause may be unknowable, but at least some of the problem (improper planting) is fairly clear. Where I do agree with Ken, is that there isn't much you can do, at this point, to "fix" the situation. Your choices at this point are to either wait and see and enjoy the tree while you can, or, cut it down and replace it (not necessarily in that order - you could plant a replacement and let it get a start before removing the existing tree). If the tree poses a danger to your home or safety, I would consider keeping an eye on it and removing it before it's likely to cause a problem.

I found a conversation that someone detailed a similar issue http://forums2.gardenweb.com/forums/load/trees/msg0117425514243.html

How does the top look? That wound looks like it was from last year. The wood is weathered grey and the callus tissue has begun to heal over. Maybe a buck rub, maybe a frost crack..., in any case it looks nasty but if the top looks healthy it will probably eventually (after a few years) heal over. Do not apply any paint or pruning sealer or anything. If the top is showing signs of stress, such as branches dying back, I would probably just replace it, especially if it is a specimen tree in your yard.

It looks like the stub is dying and nothing else. I'd still saw it off so the encapsulation process can start as soon as possible this spring. As it is, it's going to take a awhile and look bad the whole time. The broken branch should have been cut where the branch originated rather than leaving a stub. When pruning Japanese Maples, the branching pattern should be of the utmost consideration so you aren't left with a dying stub and congested branches. Mike