Aaaaargh! Nationalism, the curse of humanism. (Before I begin - I'm not American. Please read this through before yelling at me!) Quite a few people here have said they're not ashamed to be an American, but are ashamed of some Americans - and rightly so. There is some shockingly cruel, brutal and ignorant behaviour exhibited, by some Americans, and some behaviour that is nothing short of perfect, wonderful and kind. I also am ashamed of some Americans, and I am also hugely proud of some. Now take those sentences and add in any national group in the world (or ethnic group or whatever) and - guess what - it would still hold true!
I am ashamed on behalf of the human race for evil things that are done, but it seems meaningless to divide between humanity on account of an accident of birth. Don Firth - you are right; philosophically speaking, it makes no sense to be ashamed of one's own government. However, I have been ashamed of things the British government have done, because they have been done in my name. Because these things have been done in my name, I therefore feel a need to distance myself from them and to state that I do not agree with them, and I am grateful that I can do this.