The Richard Katz quote Grant posted "we don’t know really what happens when you make your infrastructure interdependent with Amazon or you place your data in the care of Google" is sure loaded with questions and considerations.
The patriot act, a topic raised in the "boundaries and barriers" thread is a big one. Another is related to the risks of losing content stored on private service sites. This blog post from Vicki Davis describes a dilemma that is quite common. Some of these services allow ways to keep copies of your content locally.
What to do? What criteria do we use to decide if a service is reliable?