Just the other day I had the need to try to run PowerShell x86 on a Windows 2008 R2 x64 machine. It took me a little time to find it.
I thought that it would live in:
Logical, I thought.. but no, it actually lives in:
I was a little amazed! It is explained though in the following article: http://support.microsoft.com/kb/926139/en-us