There is an interesting artcile about PEX at http://blogs.msdn.com/b/ukmsdn/archive/2011/02/22/featured-article-pex-and-visual-studio.aspx

PEX can be downloaded from http://research.microsoft.com/en-us/projects/pex/downloads.aspx

The licence conditions are:

"Pex and Moles are Power Tools available for commercial use for MSDN subscribers. Moles is also available separately for commercial use without requiring an MSDN subscription. Pex and Moles are also available for academic and non-commercial use."

I note with interest that it is now available to MSDN subscribers. If I recall correctly it used to be only available to VS Team versions.