The other day I attended Microsoft’s Mix 07 conference in London, primarily a showcase for what is coming in ASP.Net and Silverlight, Microsoft’s cross-platform browser plug-in.

Silverlight 1.0 is out now as a multimedia plug-in, but the focus of the conference was on the 1.1 release, promised for 2008, which includes a runtime engine for .Net languages, such as C#, Visual Basic and Iron Python.

Microsoft plugged a significant gap in Silverlight’s compatibility story earlier this month, announcing a partnership with Novell to implement Silverlight for Linux. There is still no support for non-Intel Macs, but even so, I am hearing things from Microsoft that I never expected to hear.

“You can’t have a great web client experience that only runs on 80 per cent of machines,” I was told by Scott Guthrie, the man in charge of most .Net development at Microsoft. “We need to build our credibility in that space. Cross-platform and having a quality implementation is very important.”

Can this new-found commitment to cross-platform compatibility be true? Time will tell, but I do not doubt that the .Net team is serious. Furthermore, the logic behind it is inescapable. Applications are moving to the internet, and Windows-only client solutions are in decline. If Microsoft wants to appeal to developers, it has to compete in a cross-platform world.

The real test will be how much adoption Microsoft wins for its new client platform. Adobe already has Flash and Flex, which together have spooky similarities to Microsoft’s offering. But Guthrie is dismissive. “The Flex total developer population is estimated at 20,000 developers. Our client application development base at Microsoft is in the seven to eight million range.”

However, Guthrie is looking at the wrong numbers. As an application platform, Silverlight primarily appeals to ASP.Net developers, while Flex has a server component called LiveCycle that runs on Java Enterprise Edition. It is significant that BEA has this month announced an agreement with Adobe to bundle Flex Builder and LiveCycle with its WebLogic application server and tools.

Although it is possible to mix and match, Silverlight is a natural fit for ASP.Net, whereas Flex is a natural fit for Java. According to the web monitoring site Netcraft.com, nearly 35 per cent of all web developers work on ASP.Net. That gives Microsoft a strong base for Silverlight, just as Java and partners including BEA and Salesforce.com give Adobe a strong base for Flex. Both will succeed, and like it or not, web sites that host highly visual and media-rich interactive applications will be increasingly common.