There are several things that immediately come to mind whenever I hear or read this:An armed citizenry in the United States cannot stand up to tyranny of government in today's day and age. Our AR-15s and handguns are useless against AC-130s and cruise missiles and Predator drones and F-16s! If the government wants to oppress you, they will.
1) This is equivalent to telling a child that he is banned from owning baseball bats for the rest of his life because he sucks at baseball.
2) What will this hypothetical tyrannical government use to oppress the people? The military of course! Who is the military? The people themselves! Who do you think flies those planes and pilots those tanks? Your neighbors, your friends, your former classmates from high school. I think people overestimate the willingness of the National Guard and U.S. armed forces to massacre their fellow citizens. Keep in mind - the enemy in this situation would not be "brown people" (as George Carlin put it) with a foreign culture and religion in a part of the world most Americans know nothing about - but rather the enemy would be us.
3) Finally, people making this argument severely underestimate the effectiveness of low-tech, leaderless resistance. Look at Vietnam, look at Iraq, look at Afghanistan - in all cases the U.S. military had superior technology but nevertheless got locked in a quagmire that sapped their will to fight. In Iraq, militants armed with little more than with assault rifles, cell phones, and improvised explosives did serious damage to the most powerful armed force the world has ever seen.