It's a strategic move to exploit the developer community. Microsoft is open sourcing the part that runs lightweight scripting languages like IronPython and javascript.

This would be so that programmers who create new lightweight languages can easily add support for those languages into Silverlight, enhancing its popularity. Microsoft will get that input quickly and for free.

Meanwhile the main part of Silverlight, and the part that runs the heavy duty languages C# and VB.NET, remains closed.

This is a common pattern. Also, it's not really new. Software companies have always provided source code to tools that enhance their main products. For example, Microsoft has provided the source code for product installers since around 1991.

Yes, Microsoft may be giving just enough of Silverlight to the community to get their contributions, while safeguarding more valuable parts of the technology. If such a strategy has been going on for a while, its interesting that a lot of news articles refer to Microsoft's move as as "open sourcing," which plays beautifully to Microsoft's advantage.

There seems to be some confusion about what part of Silverlight will be available under an open-source license.

IronPython is and has always been open-source, having started outside of Microsoft and being continued in the same manner from within Microsoft. Optional integration with the Visual Studio Integrated Development Environment (IDE) requires a commercial version of VS 2005.

It is not surprising that IronRuby will be supported in the same way.

The newly-announced Dynamic Language Support, an add-on to .NET, is the new part, and it enables more dynamic languages to be integrated on .NET.

It would appear that those particular features are not exclusive to Silverlight, although their availability was featured at MIX07 along with announcement that Silverlight 1.1 will support these languages along with its inclusion of a .NET engine.

As Tom Healy notes, this does not seem to involve opening any of the the Windows Presentation Foundation part of Silverlight (the formerly WPF/E bits). The supporting Expression Suite is an expensive commercial product and it will be interesting to see what the "competitive upgrade" offers will be, if any.

I think Novell's moving to targeting Silverlight, needing to make a an open-source version of WPF/E for Mono instead of the heavier-weight WPF, seems expedient. Mono and is likely to be the primary place a full Silverlight stack (less the proprietary Expression Suite and Visual Studio developer bits) will be available as open-source code.

I'm not sure how much appeal there will be to the availability of source code beyond providing reusable examples of .NET integration of a dynamic language, a matter of interest for a relatively small community compared to the number of people who will take advantage of these languages on .NET. The language implementations are basically-free supplements to the heftier commercial parts that developers will tend to have already. (The source code will also be useful in learning how to integrate the languages into other .NET-friendly development suites.)

Scott Guthrie, whose team created Silverlight and some related technologies, has a great rundown of the announcement and its content all in one place:
http://weblogs.asp.net/scottgu/archive/2007/05/07/silverlight.aspx

There's a great diagram that is used to describe the reach of Silverlight and the technologies related to it. See if you can find the parts that are announced to be open source. That will give you a sense of how IronPython, IronRuby, and the DLR fit into the overall scale of the activity.

I think it is very interesting that this was done, and there may be room for more. But mostly, Silverlight is seriously proprietary. It's not clear where they go from there, or even if they need to.