Wow, this is completely unexpected and nice. They are calling it as a developer release. Plenty of features and tools. Check it right now.
But some important points you need to know is, Silverlight 4 development is only on Visual Studio 2010. But, no problem Visual Studio 2010 can be installed side by side with Visual Studio 2008 SP1. So, get it and start play with it. Enjoy!!! with new finding and learning.