I don't understand why anyone's site is only in one datacenter. i thought the point of AWS was that it was distributed with fault tolerance? Why don't they distribute all the sites/apps across all their centers?
Weigh this against the estimated costs of your application going down occasionally. It's really only economical for the largest applications (Netflix, etc.) to build these systems.
Every day I have to build basic redundancy into my applications I wish that we could just go with a service provider (like Rackspace / Contegix) that offered more redundancy at the hardware level.
I know the cloud is awesome and all, but having to assume your disks will disappear, fail, go slow at random uncontrollable times is expensive to design around.
If you don't have an elastic load, then the cloud elasticity is pointless - and is ultimately an anchor around your infrastructure.
two c1.medium, which are very nice for webservers, are enough to host >1M pageviews a month (wordpress, not much caching) and cost around $120/mo each, effective $97/mo if you prepay for 12months at a time via reserved instances.