On a related note, if we look at corporations as nations with citizens being their employees, then actually they work in a more similar way to a government - they usually provide healthcare, define certain laws (as in, rules and dependencies within the corporation itself) and seek to make their employees happy (or else employees will go to other company). And additionally people don't get so zealously protective about their corporations as they do about nations, so maybe rather than waging wars they would choose to simply change the corporation they are in.
Well, I certainly don't say we should make a change in this direction though; it's simply a thought I find interesting, how corporations provide to their employees some of the things similarly as the government does (and sometimes provide even though the government should, but doesn't).
Corporations are under no such compulsion. Their sole motive is profit. Put Wal-Mart in charge, and they'll happily kill half the population and feed them to the other half if the accountants say it will cut costs somehow. Yeah, every now and then you get someone with a conscience in charge of a big company, but if you're going to rely on that you might as well just cut out the middleman and go straight to a "benevolent" dictatorship.
Oh, the other thing you missed is that most corpocracy scenarios (including the one that we're actually living in) aren't about corporations standing up to the government, but about them using money and influence to make sure that the government does what's best for the corps instead of what's best for the people. Giving the corps more power doesn't create a balance, it just makes it easier for them to pull the puppet strings.