I went to the VMWare virtualization conference in June 2010. What a great experience!!
After talking to several of the vendors, it has become clear to me that something overlooked seems to be the issue of security WITHIN the Cloud Providers for your data.
Terms such as "trust us.. we will keep your data safe", or "No one has ever asked that question" do not seem like valid responses to me.
For instance, who does backups, where are they kept, what kind of legal rights do you have to the data, what if data is leaked, who has the responsibility for the leak and what remedies do you have ?