If you have been developing with Microsoft Visual Studio 2010 (Microsoft LightSwitch) or Visual Studio 2010 Express you may need this upgrade without which your programs before upgrade may not load like mine did (see the message below). This upgrade installs updated Silverlight 4 Tools and brings with it a bunch of improvements noted here copied over from the source (review link).
Download the file below from the above link. Make sure you read all the enhancements you will be getting on that link.
Run the executable with a few easy steps but takes considerable time as it unwinds some and wires up some new items shown here.
Now you can go back and open your projects created before the upgrade without any problem.