Please, please everybody, before saying that the cloud is amazing let it resist the test of time. I am myself using some cloud software but there is one thing that is not yet being reflected very well, the ugly monster that lurks everywhere: Legacy.
How will AWS/RackSpace/SaaS software handle legacy data/ code in, let's say 10 years? I have no answer to that question and I bet few people have it but time is a tough mistress for computer software and data. The good thing when your data is not in the Cloud, especially when you don't control the inner mechanics of a database like SimpleDB is that you can migrate, change it the way you want at the moment you want. When you have no control over your data, well bad things could happen in the future.
Be smart, be patient.