It is a popular misconception that "I am." is the shortest sentence in the English language. That this is a misconception is typically explained by recourse to one of two explanations.
The first of these is that "I am." is arguably not a sentence at all. The crux of this argument is that the verb "to be" does not imply action, and therefore requires some kind of complement (e.g. "I am Norwegian") in order that a sentence be formed. In my opinion, this argument is not a good one, because it is far from obvious that "I am." does not imply action. One could very well claim that to "be" is in itself an action, as implied by God's words in tomed's write-up, or by the most common translation of Rene Descartes' cogito: "I think, therefore I am." It seems to me that if "I exist." is a sentence, then "I am." is equally a sentence.
No, "I am." is not the shortest English language sentence for another reason. In English, the imperative case does not require a subject. So, for instance, I need not say "You leave!", I may merely say "Leave.", and this will be a perfectly valid sentence. I cannot think of any single-letter verbs, but the first two-letter verb to spring to mind is "to go". It seems to me, therefore, that "Go." is at least the joint-shortest sentence in the English language, and may be absolutely the shortest, depending upon the existence of any other two-letter verbs that make sense when expressed in the imperative case.
I only practice grammar as a hobby, so I put this out there in the full knowledge that I'll probably be eaten alive by the grammar Nazis. I am not afraid.