Smart quotes in rich text fields?
It turns out Smartdown should do the trick. It's a Craft plugin that adds MarkdownExtra and SmartyPants (the latter of which matters here) support to Twig.

SmartyPants is a rather ancient blogging plug-in from John Gruber for intelligently handling smart punctuation. Michel Fortin ran with it in the form of PHP SmartyPants, and most recently this Craft plugin by Experience.

Note: my first answer linked to a SmartyPants Craft filter by Experience that's apparently not finished. That one won't get you very far. :)