In the XaaS world it could be your service provider closing down or some sort of security breach as happened with Sales Force or an outage which causes data loss as happened with EC2.
That's why I talked about the need for Patration, which of course everyone knows means:
“the freedom and portability to move from one service provider to another without hinderance or boundaries".
(James Urquhart has a more sensible term, which is less likely to cause arguments with English teachers, in Software Fluidity.)
Even if your provider is under an obligation to return your data to you in a usable format, it is at the very least damn expensive and time consuming if you have nowhere to go. What you need is choice, competition and portability among multiple providers.
I happen to agree with some of Christofer Hoff's predictions. I feel that 2008 is going to be the year where some of us get a refresher course on those painful second sourcing lessons.