Who IS the fat lady and why must we wait til she sings?

With respect to this question, posed in my Sunday blog on letting Wisconsin seal the fate of Dean, I can now some clarifications:

Thanks to the reader who, in answer to who she is, pointed me to viable contenders. In the first, I see the clear indication that girth may add oomph to a crescendo in a Wagnerian opera. In the second, I see that Brunhilde's girth may actually overwhelm anything and anyone who threatens to come close. I understand that both would indeed give finality to a crucial moment – be it in opera or politics.

Thanks also to the reader who suggested that I look to baseball for answers. Apparently the phrase itself is attributed to the great Yankee baseball catcher Yogi Berra, who likened a baseball game to an opera, noting, "It ain't over 'til the fat lady sings..."

All very helpful. I did vaguely recall that the phrase got to be associated somehow with baseball, but I couldn't imagine why the game would be over at the end of the singing of the National Anthem, nor why the person singing it had to be of a large frame.