You wouldn't get a business case approved without estimating ROI. But business leaders frequently aren't held responsible for achieving it. One way to enforce such responsibility is to budget for it early in the process.
When you use this practice, called budget accountability, business leaders who ask for IT projects are told, before developing their business case, that any benefits, including cost savings or increased revenues, will be incorporated into their future budgets. When a business case is accepted and the project is approved, the finance department adds a new line item to that business unit's budget.
Doing this helps in two ways. First, it keeps business and IT sponsors from inflating their estimates of the potential value of an investment. Second, because business sponsors are on the hook financially, they'll be much more engaged after a project is implemented, when they have to push the changes necessary to take full advantage of a new IT capability. They'll be sure to make the organizational and process changes required to realize the estimated benefits-and exploit those investments even further.
Budget accountability may not be appropriate when piloting new technologies, such as social networking and mobility, for which not all the benefits are known. But it should be standard for any process innovation or revenue-generating investments.