The release-to-the-web (RTW) version of Silverlight 4 is available for free download, as of April 15, as Microsoft officials said it would be earlier this week.
One important caveat for developers, as acknowledged on the Silverlight download site:
"Visual Studio 2010 can be installed side-by-side with Visual Studio 2008 SP1. For Silverlight 4 development, you will need the released version of Visual Studio 2010. Please read the known issue on installing Visual Studio 2010 if you already have the Silverlight 3 SDK installed."
Silverlight is a browser plug-in that supports multimedia content. It also is a slimmed-down, cross-platform version of Microsoft’s Windows Presentation Foundation (WPF) programming model. Each successive iteration of Silverlight includes more and more WPF functionality (and vice versa).
Silverlight 4 adds support for data binding, enterprise networking and printing, and lots of other features that are likely to make the platform more appealing to folks writing not just single-function, lightweight Web apps, but enterprise apps, as well. Microsoft's Tim Heuer has links to the full list of Silverlight 4 features on his blog.
Silverlight also is Microsoft's primary development environment for Windows Phone 7 devices, but the current Silverlight mobile dev platform is a hybrid of Silverlight 3 and Silverlight 4, not pure Silverlight 4. Microsoft officials have said that Silverlight won't be running (as an Internet Explorer plug-in, at least) on the first Windows Phone 7 devices that ship by this holiday season.
It's on to Silverlight 5... and none too early to start suggesting features....