Edd Dumbill has a blog post entitled Afraid of the POX? where he writes

The other day I had was tinkering with that cute little poster child of Web 2.0, Flickr. Looking for a lightweight way to incorporate some photos into a web site, I headed to their feeds page to find some XML to use.
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The result was interesting. Flickr have a variety of outputs in RSS dialects, but you just can't get at the raw data using XML. The bookmarking service del.icio.us is another case in point. My friend Matt Biddulph recently had to resort to screenscraping in order to write his tag stemmer, until some kind soul pointed out there's a JSON feed.

Both of these services support XML output, but only with the semantics crammed awkwardly into RSS or Atom. Neither have plain XML, but do support serialization via other formats. We don't really have "XML on the Web". We have RSS on the web, plus a bunch of mostly JSON and YAML for those who didn't care for pointy brackets.

Interesting set of conclusions but unfortunately based on faulty data. Flickr provides custom XML output from their Plain Old XML over HTTP APIs at http://www.flickr.com/services/api as does del.icio.us from its API at http://del.icio.us/help/api. If anything, this seems to indicate that old school XML heads like Edd have a different set of vocabulary from the Web developer crowd. It seems Edd did searches for "XML feeds" from these sites then came off irritated that the data was in RSS/Atom and not custom XML formats. However once you do a search for "API" with the appropriate service name, you find their POX/HTTP APIs which provide custom XML output.

The morale of this story is that "XML feeds" pretty much means RSS/Atom feeds these days and is not a generic term for XML being provided by a website.

PS: This should really be a comment on Edd's blog but it doesn't look like his blog supports comment.