The interoperable solution needs to be easier to set up and/or much cheaper than Dropbox/Facebook/etc. though. Any interoperability standard will have to be obvious to normal people to have a hope of being a real selling point and make people strongly averse to signing up for a non-interoperable service, and WebDAV doesn't seem to be anywhere near there.
I think you are missing the 'part of' part. To get an interoperable solution for cloud storage, you need a shared protocol for cloud storage, but
a) it the only thing you need.
b) I do not see why the normal users would have to be able to set up a server providing the service. Normal people do not run a mail server or know anything about POP or IMAP, but they can (sometimes with a little help from their email provider) set up er devices to work with any email provider.
If WebDAV were as wide-spread as POP and IMAP, we could have a world where, say, an iPad had a setting where one entered the URL of a WebDAV provider, a user name and a password, and the iPad apps then use that dropbox by default instead of hard coding a way to connect to DropBox.
Having an easy way for people to set up a WebDAV server at home would help getting there, but isn't strictly necessary. What is necessary is that a significant fraction of cloud storage providers agree on a protocol. For that to happen, I think those providers must feel threatened by a large provider (or maybe not; IIRC, Dropbox already has WebDAV support)
With that in place, the final step would be to get device OS manufacturers to support that user setting. That might be the largest stumbling block.
I think that in order for interoperability to happen, interoperability needs to be a visible and well known feature. If it's not, a market leader has little incentive to make it easy for people to migrate to their competitors.
But if Dropbox already supports it, like you say, maybe it'll actually happen. Now we need to find the WebDAV for universal login - OpenID in its current form is pretty clearly a failure at this point.
Yes, that is why I think it would require a sufficiently large part of the market to be afraid to be on the losing side that could make them combine their strengths.
As to that Dropbox WebDAV support: I just checked, and unfortunately, I was wrong (http://www.dropbox.com/help/62). There is third-party WebDAV support for dropbox (dropDAV.com), but that, IMO, barely counts.
