Personally, I've always thought the ideal metaphor isn't a melting pot or a tossed salad. A better one would be that of a good stew.
Think about it. In a good stew, each ingredient remains distinct, and lends its flavor to the dish as a whole. However, in return, each ingredient absorbs a bit of the broth which binds the stew together. So in the end each ingredient is definitely a part of the stew, and acknowledges this, but still remains a very distinct, separate entity. While there are faint similarities in the new flavors, you can still quite easily tell the beef from the potatoes, carrots, celery, or whatever, because they remain what they are. It's not an ideal the US has always lived up to, I know. But it is the ideal.
And dammit, now I'm hungry.
Forgive the lack of links in the original version; I'm still getting the hang of this.