Since Grunt version 0.4 (which was released 1 or 2 weeks ago) you need to install the grunt command line tool globally (if needed, use sudo before the command):
npm install -g grunt-cli
In your project location you then install the latest grunt version:
npm install grunt --save-dev
Option --save-dev will save the npm config in your package.json file, which makes it easier to install or reinstall the dependencies (using just npm install).