SAN FRANCISCO – Microsoft is using the MIX’10 Web developer conference in Las Vegas this week as a platform for unveiling new details about the upcoming Windows Phone 7 series. One detail, which could drive support for Windows Phone 7 apps, is the reliance on Silverlight as a development platform.

There are two reasons that using Silverlight as the foundation for Windows Phone 7 development makes sense. First, Silverlight is cross-plaftorm–enabling developers to create Web-based apps that work on Windows, Linux, and Mac OS X.

The trend toward cloud-based computing–and the launch of Microsoft’s Azure cloud platform, coupled with a general explosion of Web-based applications make Silverlight an ideal candidate for development. Using Silverlight means that developers can create apps without the hassle of coding to specific API’s (application programming interfaces).

The other reason that Silverlight makes sense is that developers are already familiar with it. One of the things that defines a successful smartphone platform is the extent of its apps library–just look at the iPhone and Android app stores. By building on a platform that developers already have experience with, Microsoft is making the transition to developing apps for Windows Phone 7 that much easier.

Developers can build apps on Silverlight and port them from Windows 7 desktops, to Linux and Mac OS X systems, as well as to Windows Phone 7 devices with relative ease. The convenience of writing an app once and being able to repackage and cross-market it for various platforms is a great opportunity for developers and a welcome change from having to invest additional time and effort to port apps, or rebuild them from scratch, for each platform.

“It’s the same programming model you know today,” Microsoft vice president Scott Guthrie explained to Mix’10 attendees. “It isn’t ‘Silverlight Lite’, it isn’t ‘Silverlight Different’, it’s Silverlight.”

Microsoft launched the Silverlight 4 RC (release candidate), and announced that Express editions of Visual Studio and Expression Blend will be available for developers. More importantly, Microsoft is providing a complete emulated Windows Phone 7 system in a virtual machine.

Actual Windows Phone 7 hardware won’t be available for months, but Microsoft wants (needs) developers to get into gear cranking out Windows Phone 7 apps. Creating a virtualized Windows Phone 7 provides developers with an environment for testing and fine-tuning apps without needing an actual Windows Phone 7 device.

As a side note–while Silverlight has been declared by Microsoft as the foundation of applications and functionality for Windows Phone 7 series, XNA is the designated platform for Windows Phone 7 games. XNA Game Studio isn’t quite as comprehensive in its cross-platform capabilities, but it does provide game developers with a framework to develop a game that will work for PC, Xbox 360, and Windows Phone 7 platforms with minimal changes.