So, when should you even start thinking about a CDN?

I've been working on a web app lately, and it's actually getting some decent traffic. It's mostly static assets being served right now, and it's all coming directly from our origin servers. I'm starting to wonder when it's actually time to bring in a Content Delivery Network. Do you guys look at specific metrics, or is it more about just anticipating future growth? What's your trigger point for adding that layer?