I see the current impetus to move applications to the cloud as a logical extension of moving from physical servers to virtual ones.  We have assisted several clients in a move from the physical to the virtual and there were disappointments in all cases relating to a drop in performance which took considerable time to troubleshoot largely because the virtualization provider, be it in-house or external would insist that their part was perfectly fine and that it must be the code, database schema etc.  The frustrating part is that typically the application would perform acceptably back on the physical server infrastructure.  

We have been trying to figure out adequate ways to test what a virtual environment would behave like, performance-wise and stumbled upon a new product that looks interesting in this regard.  It is aimed squarely at VMWare's offerings but they are still the most widely used in my experience.  I thought I would like to this new product here in case it helps anyone.  The claim is, verbatim "The patent–pending VKernel Capacity Analyzer enables you to achieve maximum performance of your VMware ESX infrastructure with the right amount of resources."

I also had a very interesting conversation today with Patrick Lightbody of "BrowserMob".  Basically they have developed a way of using Cloud Computing to run distributed performance tests involving thousands of browser endpoints.  In the era of web2.0 I think that is a very exciting development.