We just released v0.16.40915.5 of Pex. The major highlights of this release are Moles, a lightweight detour framework, and better support for test frameworks.

• Moles, a lightweight detour framework: Moles is a new extension of the Stubs framework: it lets you replace any .NET method (including static methods) with your own delegate.(sample outdated) 
  Want read more about this sample? here is the full walkthrough.
  The moles framework is not yet feature complete; moles does not support constructors and external methods. Some types of mscorlib cannot be moled as they interact too deeply with the CLR.
• Pex gets its own HostType: A HostType is a feature for the Visual Studio Unit Test framework that lets specific unit tests be run under specialized hosts such as Asp.Net or VisualStudio iself. In order to create reproducible test cases using Moles, we had to implement a HostType that lets tests be run under the Pex profiler. This is very exciting because it also opens the door for many uses of Pex such as fault injection, dynamic checking, etc… in future versions of Pex. When generating test cases with Pex, all the necessary annotations will be created automatically. To turn the Pex HostType on hand-written (non-parameterized) unit tests, simply add [HostType(“Pex”)] on your test case.
   
  This feature only works with Visual Studio Unit Test 2008.
• Select your test framework: the first time you invoke ‘Pex’ on your code, Pex pops up a dialog to select your test framework of choice. You can select which test framework should be used by default and, more importantly, where it can be found on disk.
  
  If you do not use any test framework, Pex ships with what we call the “Direct test framework”: in this “framework”, all methods are considered as unit tests without any annotation or dependencies.
   
  These settings are stored in the registry and Pex should not bug you again. If you want to clear these settings, go to ‘Tools –> Options –> Pex –> General’ and clear the TestFramework and TestFrameworkDirectory fields:
    
   
• Thumbs up and down: We’ve added thumbs up and down buttons in the Pex result view. We are always looking for feedback on Pex, so don’t hesitate to click them when Pex deserves a thumbs up or a thumbs down.
  
• Performance: Many other performance improvements under the hood which should avoid Pex hog too much memory in long running scenarios.
• Miscellanous improvements and bug fixes:
  • Support for String.GetHashCode(): we now faithfully encode the hashing function of strings, which means Pex can deal with dictionaries where the key is a string.
  • Fewer “Object Creation” messages that are not actually relevant.
  • In VS 2010, the package used to trigger an exception when the solution loaded. This issue is now fixed.
  • In Visual Studio when an assembly had to be resolved manually (i.e. that little dialog asking you where an assembly is), Pex remembers that choice and does not bug you anymore.
  • And many other bugs that were reported through the forums.