As @danben mentioned, there is a difference between S3 and EC2.
One thing that may be interesting for people looking to host a website on Amazon, specially if they want to start small is that Amazon started offering a free tier some months ago. Together with services like BitNami Cloud Hosting (disclaimer, I helped design it, so it is a bit like my baby :) means you can get your site on the Amazon cloud in just minutes, for basically 0 dollars. You still need to give credit card info to Amazon, but it will not be charged if you stay within the limits of their free tier.
One thing to consider too is that at the time of writing this (Jul 2011), Amazon restricts you to one IP address per server. If you need to host multiple domains, you may need to use name-based virtual hosts or some tricks using their Elastic Load Balancer (which will cost you more). But all in all, it is worth a try if you are a bit technical and want more control than what shared hosting provides you