It's day one of Microsoft BUILD, the company's major developer conference. Traditionally at these things (this is only the second BUILD, but before it there were almost 20 years of PC conferences that served a similar purpose) the keynote presentations are developer-heavy. The speakers tend to talk about Microsoft's latest developer tools and operating system platforms, do some programming live on stage (always a crowd-pleaser), and show off new testing and source control features—playing to the audience.

But today was different. BUILD's opening keynote was presented by Microsoft CEO Steve Ballmer. Ballmer was in top form. Thanks to certain videos, the man's presentation style has developed a certain reputation: over-confident, volume cranked up to 11, bombastic, and, it must be said, sweaty.

Today, he was none of that. He was composed, funny, natural, and overall tremendously likable. He was, as always, excited about the products Microsoft is delivering, but where sometimes that excitement comes across as almost scary, today it was infectious enthusiasm.

Steve Ballmer is plainly a person who believes very strongly in the products that Microsoft has built, namely the recently-released Windows 8 and Surface and the imminently available Windows Phone 8. He believes in the underlying vision, and today, in his keynote, that's what he was selling to the thousands of assembled developers in a giant tent on Microsoft's campus, and many thousands more watching streams online.

The vision once had a name—"three screens and a cloud"—but that terminology appears to have been left on the scrapheap of history alongside the term "Metro". Even without this name, that is the vision that Ballmer was selling. Three Microsoft platforms: the PC/tablet (because for Redmond, the latter is just one kind of the former), the smartphone, and the TV-connected console-cum-media player. All three are unified with a common design and aesthetic, have a (somewhat) similar development platform, and are tied together with cloud services.

That vision still isn't fully realized, but it is manifest today in a way that it never has been before. This is Microsoft's platform, this is Microsoft's future, and what it needs from developers is simple: it needs them to buy into the platform and develop applications. It needs new applications, Metro-style applications, for Windows 8 and for Windows Phone 8.

Ballmer's job today was to sell that platform to developers. He had to get them engaged and excited, and most important of all, to convince them that the audience is there; he had to convince them that if they built their apps, there would be tens and hundreds of millions of Windows 8 and Windows Phone 8 users to buy them. Build the apps and the customers will come.

How will Microsoft get those customers? A range of hardware spanning any usage scenario, from 10-inch tablets to 82-inch monster touchscreens including touch-screen Ultrabooks, all-in-one family PCs, and even powerful workstations, is a good start. This, coupled with a saturation marketing campaign, will be all but unavoidable.

For Windows Phone 8, Microsoft is betting big on Windows 8. With its common look-and-feel, the company is counting on Windows Phone 8 being the natural smartphone choice for Windows 8 users. Windows Phone 8 will be the smartphone that feels familiar to Windows 8 users, and thanks to cloud services like SkyDrive and Xbox Music Pass, it's the phone that will be best integrated and best connected.

Will developers respond? The response at BUILD was enthusiastic, but this is arguably to be expected. At BUILD, there's a certain degree of preaching to the choir—you don't generally attend a Microsoft developer event unless you're invested in the Microsoft ecosystem. The real test will be in coming months, when we'll see if developers are taking Redmond's platform seriously from the apps they produce. And the onus is also on the software giant to deliver the users it promised. Microsoft has built it, so will they come?