Windows Azure is still a work in progress, but Microsoft is already telling partners that it will be resoundingly popular with developers. The Azure team is projecting that 50,000 applications will reside in its cloud within three years, according to sources close to the company.
Salesforce was founded in 1999, and Amazon Web Services launched in 2002. Microsoft is the new kid in town, and it has a lot of catching up to do. Salesforce.com hosted 110,000 business applications as of June, and I'd suspect that Amazon Web Services is at least equally as popular.
Amazon has not released detailed usage information publicly, but it is widely used by startups. Of course, developers creating multiple versions of the same application probably inflate those numbers.
Microsoft remains primarily a software company, but I'm told that it is pitching Azure more on its merits as a business model than as a technology. Whether that pitch works as well as Redmond has conceived will be answered after it ships Azure this fall.