Let's get a few things straight about communism, socialism and modern US liberals. To think clearly about this, we have to realize that the word "communism" is being used by some of you to refer to the theory, and by others to refer to state systems that most would agree are correctly labeled as communism. These are two quite different things.
Communism pure and simple is a political and economic theory that has never actually been practiced (or should I say achieved) on a national level anywhere. As others have pointed out, Marx's original theory proposed that eventually the state would wither away and the people would control, operate and benefit from the means of production.
Marx, and later Lenin and Stalin, believed that at least two things were necessary in order to bring about communism. The first was an intermediate period of government, and indeed a powerful government, to manage the means of production until the old capitalist ways of thinking fell away, and the second was an immediate nationalization of the means of production.
(Incidentally, nationalization of the means of production, viewed by itself, is socialism. Socialism has indeed been practiced in quite a few countries, including countries that called themselves and that we would mostly agree to call communist countries.)
Numerous countries have self-identified as communist countries, with disagreement about that title coming only from academia, not because they had achieved communism but because they officially espoused communism as their political and economic doctrine and goal. With the exception of Eastern European countries that fell under Soviet influence after WWII, communism was adopted in virtually all countries that adopted it as the result of uprisings from the lower economic classes (I'm not aware of any exceptions, but could be wrong) and it was attractive to them because it promised (but rarely if ever delivered) a more fair distribution of the benefits of production.
Many young American liberals in the 1960s into the 1980s thought that communism was a good idea, and indeed there were still a few openly "communist" university professors at least into the early 1980s. But even the most liberal adults that I know now believe quite firmly that communism as a political and economic theory is unworkable in the real world, at least on any sizeable level. First, the old capitalist ways of thinking do not fall away, meaning that the strong central government can never fade away (which is fine with the leaders, anyway, as others here have pointed out). Second, as someone here (who doesn't seem to like liberals very much) pointed out, capitalism (or at least modestly regulated capitalism) beats socialism in practice. (The thing that our anti-liberal friend doesn't realize is that modern liberals in the US agree that capitalism as a practical matter works better.)
I recognize that there is a so-called Communist Party in the US, but it is truly a small fringe group. Virtually no one on the American left accepts communist theory as a wished-for goal. As for socialism, it's popular on the right to accuse Obama of being a socialist, but if you want to use that term correctly, you need to point out to me what means of production he has urged the government to seize, own, and operate on an ongoing basis. We can argue about GM, but a real socialist would scoff at that since the government owns no part of it any more.
When the American right accuses Obama and Democrats of being socialists, what they are really complaining about is the fact that Democrats (and modern liberals generally) believe that progressive taxation is fair and just, and believe that the government has a role to play in helping the most needy and vulnerable in society. These things appear to the right to be similar to or derived from the theories of communism and socialism. But the reason they appear that way is because liberalism, socialism and communism are all, in significant part, theories of justice. And virtually any theory of justice includes to some degree the concept of society helping its neediest (which itself implies progressive contributions in some form).
The only theory of justice that does not contain any grain these elements is, by definition, "I got mine, so go away."