>> My experience with EC2 is that you get more RAM for the buck when compared to Rackspace and others, but IO to disk and CPU is sub-par. As a result, I tend to prefer Rackspace who are also big players in the open source space.
Seconding this. And I'd recommend a hybrid solution if it's possible. Dedicated db server with some cloud webheads for your front end. I really think that's the best bang for your buck. Has anyone tried the Amazon RDS or the RackSpace R2 servers? I'd really like to know if they provide any advantages over just straight up hosting your database on another cloud instance.