a) A large portion of AWS hosted stuff probably doesnt need that level of sudden, burst scaling
b) With something like SoftLayer/IBM you can scale physical servers, usually within 30 - 60 minutes
c) If your burst scaling requirement are temporary, if you are located in a decent DC, you can probably spin up some infra in AWS and access your physical stuff over a private network connection and get the best of both worlds.
As always, use what's best for your environment.
