It’s easy to say “ah, we’ll just run some cloud management software on a bunch of machines,” but it’s a completely different matter to uphold the premise of real-time resource availability. If you fail to provide resources when they are needed, the whole paradigm falls apart and users will start hoarding servers, allocating for peak usage instead of current usage, and so forth.
– Thorsten von Eicken

(Via the RightScale Blog, interesting read!)