I don't know why I didn't think of this before (downright embarrassing, actually), but when I post a photo from Flickr using their "Blog This" function I always have to go back and clean up the post because the STYLE block html tags are stripped, leaving the CSS dross in-between at the top of the post.
The communityserver.config markup! Obviously the <Style /> tag was not supported explicitly in the cs.config acceptable markup list. Adding it fixed me up, preserving the STYLE tags when posting from Flickr and nicely formatting the photo description text with no need to go back and clean up the post.
Geez.