This statement is false. AZs map directly to physical facilities. If you and I both ask for instances in us-east-1b they will have lower ping times than if one of us has an instance in us-east-1c.
Disclaimer: I used to work for AWS.
"Q: How can I make sure that I am in the same Availability Zone as another developer? We do not currently support the ability to coordinate launches into the same Availability Zone across AWS developer accounts."
http://aws.amazon.com/ec2/faqs/#How_can_I_make_sure_that_I_a...
If AZs map directly to physical facilities, how does that FAQ make sense?
We're straining to read the tea-leaves here, rather than AWS just telling us what they've built.