Many interesting responses and questions in this thread, don't feel like doing all of them justice.
I think you raise good questions.
And I think many of the traits that are casually thrown around for introversion are actually more temperament-oriented, or sticking to dichotomy's in mbti, are combo's of I plus F, or I plus T, etc. Simple example (and I have no doubt that there will be exceptions to what I'm about to say.. ;-) -- IxTx's will probably be more likely to say they can go days without seeing or interacting with other people, and are fine with that. IxFx's won't be able to relate to this as well.
And all of the 'where do you get your energy from' definitions can get pretty muddled and situational IMO. Based on some of the statements regarding this in here, I'd be an E. Which I'm not. ;-)
I thought TrueHeart's post was getting to the meat of a general Introversion trait/definition that could be true for all I's. More along the lines of cognitive processes and decision-making and data gathering in the brain - most easily observable in childhood before the individual matures and starts balancing out. Extroverts reacting to external stimuli immediately (questions directed to them, behaviors others exhibit towards them, conversation stuff, etc) and also initiating many things. Introverts will have a delay before reaction, where they do their little assessments/ computations/analyses/observations/deliberations/value-checks in their head first, before acting externally. (this delay could also explain why in general excessive interaction is physically tiresome for I's -- stuff has to go through their internal filters first before being externalized, and externalizing obviously isn't the dominant trait of the I.). Of course this could end up being an equally muddied definition, but oh well. ;-)