I've been thinking about how to write logical statements in XML. Apparently there are two (or more?) existing XML subset for rules:
RuleML
R2ML
These appear more structured towards business logic, which is interesting to me, but I'm thinking about something simple, like ferm (for easy rule making) but in XML.
There is also MetaL - an XML programming format, but that seems to complicated for what I'm looking for as well.