Wired News reports that while the number of hotspots is growing quickly, the number of users isn't: John Yunker of Pyramid Research notes that it's really hard to figure out how many people are paying for hotspot use seeing as few of the operators release customer numbers. He also says that it'll be tough to make money from hotspots as a standalone business. He sees more potential success for companies that may also operate other telecom networks.
I think the market will have to reach a critical mass of hotspots that a subscriber to any service can use. Until then, only a subset of business people will be willing to pay for it. The T-Mobile/iPass announcement may contribute to that critical mass through aggregation, but only for large enterprises that use iPass.
Another story has a few more details from the In-Stat/MDR study that the Wired News story cites. A large majority of hotspot users--71 percent--say their companies pay for the cost of access. And more than half the people who say they choose a venue based on Wi-Fi say they also choose the free location over the one that costs.