Here is the Hoff podcast with Gary McGraw, its a great interview and they hit on a ton of interesting stuff. Hoff talked about looking at the cloud as Infrastructure (routers, switches and such), Infostructure (apps, data and such) and that we have a reasonably good idea on how to try and secure these. Sure there are issues, but there are various ways forward.
But what worries Hoff is the Metastructure (identity, policy and such). This is all new, we don't understand the issues yet, the tools are new - this is where the challenges and opportunities lie for the Cloud. I have thought for some time that the biggest enabler for security in the cloud is decentralized policy, I guess we'll see.