Every day, it seems, Microsoft is incrementally adding to its interoperability message. Tomorrow, as Nokia blurted out this morning, its regularly scheduled increment is a demonstration of Silverlight on a mobile platform other than Windows.

Today, Nokia provided something of a spoiler for tomorrow's keynote address at Microsoft's MIX '08 convention in Las Vegas: The two companies will be demonstrating Silverlight, Microsoft's runtime environment for rich graphics and functionality, running on the Symbian S60 mobile platform.

It will not be Silverlight's mobile premiere -- last April, Microsoft demonstrated Silverlight running on Windows Mobile. But it will be a major demonstration of the flexibility of the principal challenger to Adobe's Flash platform, whose video distribution has become all but ubiquitous.

An extension of Silverlight to Symbian S60 would give the platform access to an estimated 7% of the mobile phone market, with 77.3 million total units shipped last year, according to statistics compiled by Symbian Ltd.

Microsoft is saying nothing more on the subject, deferring to tomorrow's keynote for details. But Nokia did quote Microsoft's newly promoted senior vice president for the Developer Division, S. Somasegar, as saying, "We want to make sure developers and designers don't have to constantly recreate the wheel and build different versions of applications and services for multiple operating systems, browsers, and platforms."

Silverlight contains a subset of the .NET Framework, joined with the culmination of the Windows Presentation Foundation/Everywhere project, with the objective of placing a rich but small runtime executable on client systems for exchanging graphics and interactivity messages with servers. Though Silverlight does couple with browsers, it's designed to operate independently as well, so Silverlight-driven applications could be installed by mobile phone users as separate entities rather than bookmarked Web pages.

Microsoft's and Nokia's move will follow last November's unveiling of an independently produced version of the .NET Framework for S60, by South Africa-based Red Five Labs.