Opening Double Quote after 3 hyphens converted to double closing quote

[PetesGuide]

The string:

<p>I also remember the line in "Casey at the Bat" when he struck out---"There is no Joy in Mudville!!!"</p>

is converted to this in WordPress 3.3.1:

<p>I also remember the line in &#8220;Casey at the Bat&#8221; when he struck out&#8212;&#8221;There is no Joy in Mudville!!!&#8221;</p>

Seems to be what should be an opening quote that follows an Em dash is always converted into a closing quote.