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The trouble is that the RSS standards disagree (or don't say) whether they allow this, and how it should be handled (should the HTML be included directly, or XML escaped). Mark Pilgrim wrote an excellent summary of all the nasty compatability problems with RSS:
I would recommend that you include HTML (if that makes sense for your content), and escape the HTML. That seems to work best in my experience.
On a related note,....I am definitly experiencing the inconsistencies between RSS readers. I checked my feed with [validator.org...] ...and it passes, but Pluck doesn't like it.