title: Why is StyleCop SA1305 not respecting the allowed prefixes list in VS2010 (or MSBuild)?
body: <p>I just upgraded a project from 2008 to 2010 Beta 2 and StyleCop is now reporting SA1305 (Hungarian notation) warnings on variable names with the prefix 'is'. 'Is' is definitely in the list of allowed prefixes.</p>
  
  <p>Is this a known issue? Has anyone else run across this problem? The code was definitely compiling without any warnings in 2008.</p>
  
  <p><strong>Update</strong>: It turns out that this can work as expected in Visual Studio, but then fail through MSBuild. See the answer below for why.</p>