We actually moved from a datacenter to EC2. The problem is we all live in the Bay Area, where data centers cost a lot of money. We can't afford to hire a hand on person in a cheap place, so it turns out EC2 (and other cloud solutions) are just cheaper.

Also, one thing that is nice with EC2 is that we can spin up a lot of big iron when we need to absorb new kinds of traffic. I can't do that in my own datacenter.