Have you been noticing more and more people answering a question with "I am" and the first time you hear it you think, well that sounds cool. Did you know that "I am" is actually the shortest complete sentence in the English Language? It may also be the shortest title of any article you read this week. That is not to say that it is necessary a good answer to a question unless it is stated with a proper tone.
I am could have a question or it could be an answer. It could be a very positive answer or a quizzical question. It could also be a snide remark or a number of other things. Stop for a second and say, "I am" in several different ways. See what I mean? You should there before add this to your vocabulary of conversational phrases, but only in a limited sense, as you do not want anyone to become offended and think that "You Are not" a very good friend. I am, too!
Indeed you may very well be a wonderful, personable, well-intentioned person, I do not doubt it. But make sure you are making the right impression and listen closely when someone else misuses that phrase and see how silly it sounds when they do? When someone says "I am" they may tell the truth or hiding the truth depending on how they say it.
Brevity has its place and is not always the best policy. JP Morgan once said "Say little, know a lot" and this makes a lot of sense in many human endeavors, however it may not be the most polite way to play your personal affairs.
If you read a transcript back, which has the sentence; I am in it; it could be read that you are making a statement to someone and they are saying; "I am?" rather than answering it in the affirmative; I am! You see the difference.
Usually I find some people say this to be cool, others use it tactfully when they do not wish to reveal information. Always call the person on it, if you see that their body language does not match the way they use those words. Trust is too hard to come by to allow game playing tactics of this sort. Think on this in 2006.