Daryl Plummer an analyst at Gartner has written a great piece on what he's calling, On-Demand Overspending. I love the term. Plummer is quickly becoming one of my favorite Gartner analysts, a man who isn't afraid to speak his mind on the issues surrounding cloud computing. His previous Cloud Overdraft protection concept also does a great job of outlining some of the problems in "cloud bursting" and how that can lead to run away costs.
In his most recent post aptly titled "Cloud Elasticity Could Make You Go Broke" he says "Some research done by Onecompare.com was highlighted in a web article recently citing how mobile customers in the UK were overspending on their mobile plans because they had been sold plans smaller than their actual needs. These users either talked using more minutes than they expected they did, or talked beyond the number of planned minutes they had paid for. The same will happen, both intentionally and unintentionally with cloud services."
His post is worth the read.