As discussed in the comments of the previous post on this subject, a commonly supported export format is an important first step, but it is not enough. Here’s what else is needed.
1. The export must be automatic. It must be done by the software, and kept up to date. The user must not be required to do anything to always have a good backup.
2. The backup must be stored on a static server, not on the vendor’s network. There should be a default backup service provider, but the user must be allowed to specify a different one. Apple, for example, could do this as part of their dot-mac service. I believe other companies would line up to provide such a service.
3. The user must also be able to remap their sub-domain. The user should have full control over the domain for the site, and it should not require any help from the vendor to map it to different server. An alternate, though not as desirable approach would be to allow a permanent redirect, again triggered only by a user action. The problem with this approach is that it requires that the vendor’s server be working in order for it to work.
With these three things, in addition to a common export format, even in an extended outage, the user would be in full control of his or her content. Users should insist on this level of support from vendors, or take their business to vendors who provide it.
Having been a vendor myself, I believe this kind of system will allow vendors to relax too, in addition to the users. When you have an outage, the users will have the power to move, and no doubt some will, but it takes the pressure off the vendor, and gives them time to fix the problem.