I think you need to distinguish two cases:
- A SaaS supplier is supplying a quasi-generic service. It's conceivable the data could be transferred to an alternative supplier, and a supplier could promise to make the data available in a form that could be used by that supplier.
- A SaaS supplier supplying a unique service. There is no practical alternative to the supplier other than creating your own data center. By the time you have done this, you may no longer be in business.
The question you ask normally comes up in the context of a company considering using SaaS services. In these cases a prudent company (as part of its business continuity plan) needs to (a) assure itself of the financial viability of the supplier (interesting that most people answering this question see this as the main risk), and (b) assure itself that the supplier has an adequate business continuity plan which will assure services in the event of all major risks. (For example, if a data center has a fire and has to be shut down temporarily, is there an alternate. Is it on hot standby? Is data duplicated? How much data could be lost? Can network traffic be re-routed? etc.)
Of course, the customer also has to worry about network connectivity issues: the supplier may be in business but unreachable. And (in cross-border cases), political and regulatory risks.
The concerns for a SaaS supplier are in fact not that different from any other provider of outsourced critical services or products. (If you have assemble custom flanges and custom grommets to produce widgets, you will be in trouble if your supplier can't supply you with your flanges for whatever reason.)
Interestingly a concern for a SaaS supplier with a few large customers is the financial viability and business continuity of its customers. Failure of a major retailer sometimes results in the failure of its suppliers: not only are the suppliers left with large unsecured debts, they are left lacking a major part of their distribution chain.
Jan Husdal writes an interesting blog on issues of supply chain business continuity, although I don't think he has covered SaaS issues specifically.
One indicator to watch for the future may be the requirement for a supplier to have a business continuity plan audited to a recognized standard (e.g. BS-25999). It may be that we will see business continuity standards propagating in the way that ISO-9000 standards propagated as each company pushes back certification requirements to its critical suppliers.
Good luck with your thesis. You've chosen an interesting topic. You might also want to ask your question in the Disaster Recovery Journal group on LinkedIn. It's the only really active discussion area on Business Continuity Issues I've found.