Ask for a portfolio, and then review it with your team. That takes care of posers and people who "don't handle interviews well."
Other than that, I'd present them with something relatively simple to mock up and a laptop and say 'have at it.'
Maybe ask them what they like most about web design today, and what they hate the most. Ask them about their opinions about what is on the horizon (HTML 5, IE 8, Chrome, etc) to see if they keep abreast of what's coming out.
Ask them if they have a favorite JavaScript framework and why. Maybe have them code something in JS a la the [in]famous fizz buzz problem.
