The best talk I ever saw in this area was by Paul Strong from eBay (they obviously have a very strong requirement for both scalability and consistency). He talked about how they eventually ripped out all transactions and stored procedures from the RDBMS layers and built their own giant, cross cluster, consistent system to make the zillion customer/auction problem finally managable.
For the less giant problems, I think Amazon could charge a premium for an API-managed RDBMS solution, they have all the tools for making one in place (as opposed to requiring you go build it yourself with EBS and EC2 nodes).
add-slave-node() ...
Scaling out to a dozen DB servers that master/slave their way to scalability is no fun, but it's solved. The problem is that you're currently required to do it yourself. It would rock to be able to outsource that to the cloud.
I want to toss my ASP.NET application up into the Microsoft Cloud, where it will figure out how many webservers it needs to spread itself across and how many database servers it needs to fire up to handle the load it's seeing. And I want it to pretend like it's a single webserver talking to a single DB instance on a single box.
Say what you will about Microsoft, but they have the skills to pull that off. I sure hope they're working on it.
Unfortunately brains don't scale, so you're better off using one huge brain, like Michael Stonebraker.