If reliability is that critical, you need multiple data centers. This is far easier to implement with EC2 than by building out hardware.
Also: Most downtime is caused by bad code deployment, poorly-conceived network or system configuration changes, and sysadmins with fat fingers. Do you really think your hired talent is going to be better than Amazon's hired talent?
Sorry to be rude, but that is a bit incendiary of a comment. However, given stats for the past 12 months, I can show that my availability for the 2 racks I manage, is in fact, higher than AWS. Thanks for the compliment!
Also: Most downtime is caused by bad code deployment, poorly-conceived network or system configuration changes, and sysadmins with fat fingers. Do you really think your hired talent is going to be better than Amazon's hired talent?