It's a bit curious to me that most implementing cloud computing are struggling with the concept of integration, a topic that's near and dear to my heart. More surprising they are doing this as if integration itself was a new topic.
The issue is one of context, more so than technology when considering that we've been doing integration well for over 15 years, and thus it's just a matter of carrying those concepts and technology to the world of the cloud. However, as it was back in the 90s when I wrote the EAI book, many are approaching cloud integration as if we are starting over. That's a big mistake.
What's key to remember about integration, cloud or not, is that it's actually about the free flow of information between systems that deal with information differently. Thus, you have to adjust information as it's transmitted between systems, or how each deals with system semantics. For example, in moving information from SAP ERP to Salesforce.com, at some time you need to deal with the different ways that each structures data around the concepts of customer, sales, inventory, etc..
Not a ton changes when dealing with cloud computing, other than the fact that you may be dealing with systems that are outside of the firewall, and not under you're direct control. However, they also typically provide well-defined and easy to use interfaces, or APIs, which allow access to core information or services. Indeed, I would consider it much easier to connect and integrate existing SaaS and IaaS clouds than traditional enterprise systems.
The core message here is that we need to learn from the past, and don't assume we're starting from scratch when dealing with cloud computing. The patterns, the technology, the problems, and the solutions are largely the same.