After much fiddling around, this is the solution I’ve ended up with that uses Grunt, Node, Watch, and Sass:

• Install Node: http://nodejs.org
• Create a package.json: NOTE: I froze versions of my dependencies. You may want to use ~ instead.
  
• Install/update node packages: npm install
• Create a Sass directory in your www: mkdir www/sass
• Add an SCSS file in your www/sass
• Add a Grunt config script:
  
• Run Grunt (and it will keep rebuilding any time a change to app.scss or your www/* files happens): grunt
• Open the Xcode project: open platforms/ios/YourAppProjectName.xcodeproj/
• Run the iOS simulator