I prefer to keep such comments in the code, not in the commit message. It is highly unlikely that anyone coming to do some changes to that part of the code would go and read a year-old commit message that he isn't even aware exists.
Some commits even feature implementation details, and those I agree with you sometimes it makes sense to include them in the code. However most implementation details replace old ones, and this is transition should be explained in a commit since this is the perfect place for it.
Here is an random example from the linux kernel, which is guaranteed to do what I described: http://goo.gl/QdKNnJ