From the looks of things, however, there is a significant difference between the Oracle and MySQL offerings. MySQL is a DB limited to a single machine. They talk about the ability to dynamically scale the machine (which sounds just awesome) from small to extra large based on requirement, but not about multi instance databases.
Oracle, however, does have this ability, and it is supported on EC2. So you can increase you database horizontally, rather than vertically. At least, that is how I read things.
I found this to be extremely interesting.