Just to start some discussion on the dismissal of instant scalability: "Almost any company worth a damn can bring online a server within 24 hours, even budget companies. When have you actually needed turnaround time faster than that? If you did, maybe you should read up on capacity planning."
Realistically it is often very hard (impossible) to predict how viral something new is going to go. If some "superstar" grabs onto your app and publicizes it in front of 10x your normal audience, 24 hour turn around may not be fast enough.
That's a good point, and I think in those situations the cloud will fall over just as easily. A lot of that will come down to your actual applications architecture and how well it can scale.
Realistically it is often very hard (impossible) to predict how viral something new is going to go. If some "superstar" grabs onto your app and publicizes it in front of 10x your normal audience, 24 hour turn around may not be fast enough.