The repeating table column's xpath is:
/dfs:myFields/dfs
My attempts at the preceding syntax have been based on this syntax:
preceding-sibling::NodeName[1]/elementname
or
current()/preceding-sibling::NodeName[1]/elementname
both raise the same error in the conditional formatting designer.
"preceding-sibling::ContactCategory/lu_Group" does not point to a valid location path of a field or group.
A good article from Greg Collins http://www.infopathdev.com/blogs/greg/archive/2005/06/13/Using-XPath_2700_s-_2700_preceding_2D00_sibling_2700_-Axis-Correctly.aspx