I can see why they chose MySQL, but I would gladly pay for Oracle in the cloud as well. Plus, Oracle can scale up to a much bigger machine before you have to start scaling horizontally.
edit - 5.1 (recent releases) scales better mostly due to InnoDB 1.0.4 which incorporates a number of patches from Google (Mark Callagan), Percona and more.
I'm pretty sure they're using MySQL because it's popular, not because it scales. I figure when people hit 10 million rows in a MySQL table, they start looking at sharding/partitioning instead of buying a bigger machine.
However, if you're going to go with "big" database servers, one should use Oracle or PostgreSQL since they can actually make use of the memory & cpu cores.
Amazon did a demo at the Oracle Open World a few weeks ago and it looked promising.
http://aws.amazon.com/solutions/global-solution-providers/or...