"Considering that the wealthy, the middle class, and much of the poor (who still drive cars, have air conditioning, and posses multiple TV's in the US) possess material wealth of a manner and quantity that didn't exist 100 years ago, who did they steal all that wealth from?"

You're certainly right in suggesting that the organization of matter into forms humans desire is the creation of wealth (the poster you are responding to got that wrong). And you are certainly correct that we have more stuff in the USA than ever (See Cato Institute's Brink Lindsey's "The Age Of Abundance").

But, as financial prospectuses always say: "Past performance is no guarantee of future results". You can't conclude from the poor having more stuff now that things won't change dramatically in the near term unless we actively try to reform a winner-takes-all social ideology. Reforms might include instituting a "basic income" to keep the market working (funded by a wealth tax perhaps or other means), helping gift economies prosper like GNU/Linux by more supportive laws like shortening copyright length to a few years or limiting its scope to only commercial transactions, and helping people be more productive locally in various ways where they keep or share the results of their own production from home 3D printers or neighborhood TechShops or local farms by helping individuals relearn productive skills and have access to the resources they need to do that.

Also, while we have a lot more stuff, so much that we are getting buried in it in the USA and people make movies like WALL-e that don't even sound too far fetched, we also have a lot more cancer (half of which comes from environmental causes) and a lot more risk of global Armageddon from nukes, bioweapons, killer robots, and global financial panics. We also have a lot less species and less nature. One can argue if that was a good trade overall, but in any case, it is a done deal. But here is another way to measure progress than Brink Lindsey's approach:
    http://www.rprogress.org/sustainability_indicators/genuine_progress_indicator.htm [rprogress.org]
"We believe that if policymakers measure what really matters to people--health care, safety, a clean environment, and other indicators of well-being--economic policy would naturally shift towards sustainability. Redefining Progress created the Genuine Progress Indicator (GPI) in 1995 as an alternative to the gross domestic product (GDP). The GPI enables policymakers at the national, state, regional, or local level to measure how well their citizens are doing both economically and socially."

Is Bill Gates lobbying for any of that? No, he is busy moving his R&D to India, trying to get more staff under H1B visa indentured servitude to get the most for the cheapest labor costs, trying to pay less taxes to support those workers that improved productivity from software makes redundant, and likely trying to extend the scope and duration of patents and copyrights he owns -- at least, until, like the Grinch who stole Christmas, he has a change of heart. Then, his vast talent and skills could help renew our society, physical infrastructure, and our ecology, like happened at the end of the movie WALL-e (if you stayed for the credits):
    "WALL-E Credits"
    http://www.youtube.com/watch?v=p6wgicmUAfw [youtube.com]

The issue is, where do we go from here? Mainstream economists ignore the value of social capital, like intact extended families and intact neighborhoods. Brink Lindsey ignored social trends, for example, how the USA has one of the lowest scores on life expectance and childhood happiness of the industrialized nations, and how hundreds of years earlier the natives had a socialist economic system that was working so well that Benjamin Franklin borrowed lots of ideas from the Iroquois Confederacy to write the US Constitution. They also ignore systemic risk; Brink Lindsey's book written a few years ago might have been different (if he was honest) if written today after the economic crash. They ignore the long term implications of increased automation and better design in the face of limited demand for stuff, because "the best things in life are free or cheap", and the implication that with better robotics and AI there will be less need for workers, meaning more unemployment and worse jobs (from increased competition) over time. Yes, that means *you* in twenty to thirty years will have a tough time finding a job. Cabbie? Robots drive cars better than you. Surgeon? One can do the work of ten with robots. Cashier? RFID. Product designer? Fancy software will make design easy for people with little skill. Nurse? Better medicines and robots will allow a few nurses to do so much more. And so on. See Marshall Brain's Robotic Nation.
    http://www.marshallbrain.com/robotic-nation.htm [marshallbrain.com]

Again, you confuse financial wealth (potentially zero sum) and physical wealth (potentially infinite). Also, your introducing the notion of "stealing" is a strawman argument. One can have vast inequity and lots of suffering of people without "stealing" if the social contract is written a certain way. What was the recent multi-trillion dollar bailout but a form of legal stealing from the population and giving it to wealthy speculators?

But, it is true, much has been stolen in a sense of might makes right, like perhaps the very land of the USA from the natives?
    http://www.historyisaweapon.com/defcon1/zinncol1.html [historyisaweapon.com]
"""
The Indians, Columbus reported, "are so naive and so free with their possessions that no one who has not witnessed them would believe it. When you ask for something they have, they never say no. To the contrary, they offer to share with anyone...." He concluded his report by asking for a little help from their Majesties, and in return he would bring them from his next voyage "as much gold as they need ... and as many slaves as they ask." He was full of religious talk: "Thus the eternal God, our Lord, gives victory to those who follow His way over apparent impossibilities."
"""

I've read that Manhattan Island, the granite bedrock of US capitalism itself, was purchased under false pretenses from some people in a neighboring tribe who did not live on the island.

You're right that we may disagree on 1000 assumptions, but the thing is, thirty years ago I used to talk a lot more like you do now. It has been probably literally 1000 assumptions that I was taught in school that I have had to unlearn. It is a long path. You are starting down it by being willing to make your views known in public in a forum where they can be commented on. That is a brave start.