Peter Wayner at Infoworld published a good overview of Cloud offerings from Amazon, Google, AppNexus, and GoGrid. The main takeaway: Cloud Computing is as nebulous as it is cumulus.
The first surprise is that the services are wildly different. While many parts of Web hosting are pretty standard, the definition of "cloud computing" varies widely. Amazon's Elastic Compute Cloud offers you full Linux machines with root access and the opportunity to run whatever apps you want. Google's App Engine will also let you run whatever program you want -- as long as you specify it in a limited version of Python and use Google's database.
The services offer wildly different amounts of hand-holding, and at different layers in the stack. When this assistance works and lines up with your needs, it makes the services seem like an answer to your prayers, but when it doesn't, you'll want to rename it "iron-ball-and-chain computing." Every neat feature that simplifies the workload does it by removing some switches from your reach, forcing you into a set routine that is probably but not necessarily what you'd prefer.