Jafo..As for short 'sentences', often a response to a specific question bypasses traditional subject-predicate-action syntax....'see Spot run'...and therefore 'I' could very well be a legitimate 'complete utterance, or sentence...all 'related' intent/content remaining implied...
That is what I was thinking... though in the case of I, in the cases where people should properly use I, they usually say me.
Who deserves this million dollars I am about ready to give away? 
Me? Though what is implied is "do I? I seem to remember an old rule that bows to overwhelming nonconformity to some language rules... it is how language evolves after all. A lot of our common contractions went through that process...
