I'm currently hosting with softlayer and running the following:
4 super fast http servers on a load balancer with 1000 connections (costs a fair amount) - apache 2.0
1 fast mysql server (the server is starting to struggle but i'm concerned about expanding this because clustering mysql is very complicated in my situation) - mysql 5.0
I was looking at amazon s3 and it said it would only cost me $2,500 for my needs which is not only considerably cheaper but i think theirs would be even faster than softlayers with their network and the fact that there wouldn't be hundreds of mysql connections per second as there is now.
Money is not really an issue, I'm more concerned about flexibility and in case i get a ddos attack that i can handle it since softlayer doesn't provide ddos protection (cisco guard) on load balanced IPs and all IP requests to the server show as the load balancer IP.
I've never seen the new instance sizes for EC2 but that seems to make it a much better idea imo.
I looked at it before they had that option and it was definitely a dumb idea, more expensive for bandwidth with less processing power. I might try out one of those large instances just for running tests though.