When I went to watch PRISM video, it prompts and told me I do not have latest silverlight, even though I was using SL4. Regardless I went and downloaded the SL exe and ran it. Now after installing the SL component my development box kept failing that I do not have Silveright developer runtime. I figure the download I got to watch video messed up my development box and I was right.
A quick bing got me to the following URL
which tells the same. There are two versions of SL distributable, one for developers and one for the end users. The one I downloaded was end users, which in turn overwrote my developer version. Above link also have link to download location. I downloaded the exe from the link and now everything is green and peachy.