I went in search of the definition for the Directory/@Id which targets %WINDIR%\system32, and found a comprehensive list at http://msdn.microsoft.com/en-us/library/aa372057.aspx (full disclosure: I found it as a link from the WiX tutorial at http://www.tramontana.co.hu/wix/lesson1.php#1.2 ).
In my opinion, this list would be a great addition to WiX.chm, with a reference to the MSDN article to get the most up-to-date information.
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Not a bug. Opened feature request at http://sourceforge.net/tracker/index.php?func=detail&aid=1990232&group_id=105970&atid=642717.