Nearly two years ago, I made a prediction that Atom would replace WebDAV. At the time I was even working on revising WebDAV and just beginning my involvement with Atom standardization. I told this prediction to Cullen Jennings and documented it in a note dated 5/9/06. "Replace" is a fuzzy term, because they're not equivalents, but here's what I see today.

• Google turns Atom, RSS and AtomPub into GData and uses that for blogs, calendars and task lists for starters. Google does not use WebDAV as far as I know.
  
• Microsoft is unifying its developer platform protocols on Atom and AtomPub, and using those protocols for unstructured application storage, e.g. photo albums to blogs. (h/t James Snell. Hey, I used to work with David Treadwell, haven't talked to him in years.) WebDAV support in projects like Exchange is downplayed.
  
• IBM has a bunch of projects using Atom but it has a bunch of projects period and I haven't seen strategy announcements about either standard.
  
• Apple uses Atom and RSS in quite a few applications, although it's also using WebDAV and CalDAV on .mac and in its calendar server.
  
• Blog service sites are ubiquitous and all use Atom or at least RSS. File sharing sites or other WebDAV public services are rare. Photo sharing sites are more likely to use Atom than WebDAV by a long shot.
  

Were I to propose CalDAV today it would probably be CalAtom -- some things would be easier, some harder, but it would catch a wave instead of drifting in the tail of something that was never much of a popular wave. Oh well, we needed something then, and WebDAV gave the most leverage at the time.