Is Redactor's default config saved as a .json file anywhere? When I need to make one tiny change to the default config it would be handy to be able to duplicate the current default.json file and just change a one thing instead of building a duplicate from scratch.

It's a small thing, but it would be a nice time saver. I've had no luck finding it, does anyone know if the default is saved somewhere?
As @Brandon said, the defaults are hardcoded in redactor.js so there isn't a default.json file, but if there was, this is what it would be (from Craft 2.3.2615):

{
    buttons: ['html', 'formatting', 'bold', 'italic', 'deleted',
    'unorderedlist', 'orderedlist', 'outdent', 'indent',
    'image', 'link', 'alignment', 'horizontalrule'],

    formatting: ['p', 'blockquote', 'pre', 'h1', 'h2', 'h3', 'h4', 'h5']
}


Since the above code could change on occasion consider checking directly over at the Redactor site:

Buttons defaults

Formatting defaults