I thought I'd post this, mainly so I wouldn't have to bother Stuart again. If you have built an MSI and need to verify the contents to ensure that your automated build is functioning correctly and you have WiX installed, simply open up a Command Shell and tap out the following:
%WiXDir%\dark.exe -x c:\%OutPutDir% MyInstaller.msi c:\%OutPutDir%\MyInstaller.wxs
where %WiXDir% is the path to your WiX install directory and %OutPutDir% is the path you want to dump the contents to.