You probably don't get on the phone with Dell. There's a large area in between EC2 and racking your own boxes in your own datacenter. You can buy dedicated machines from OVH or similar firms with lead time of a few days, and it's a very rare firm that can't predict their load a week in advance .. even for a fast growing company.

Look at it like this; GitHub mostly ran in their own datacenter using AWS only for spillover capacity. They scaled just fine. Virtually no apps have the problem of runaway growth that they can't handle without instantly provisioned resources.