Whoever solves the "scalability or consistency: choose one" problem will make a boatload of money. From what I can tell this is a very hard problem, so software that makes it seamless to operate at multiple levels of this tradeoff curve -- perhaps at the same time, for different parts of your data -- would also be a big win.
I think people are doing it, they are just not slicing it up in a pay-as-you-go format for end users.
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).
In addition to a cloud-hosted solution, packaged software (along the lines of Hadoop) would be great too. Not everyone wants their corporate data out in the cloud, but I suspect they would all be happy to skip writing one-off "fix RDBMS scalability" layers.
It is a very difficult problem. I worked for a short time for a company that was trying to solve consistency, distributed redundancy, and scalability. It succeded consistency and redundancy, but failed because transaction chatter caused it to fail at scaling. I think that within an application or enterprise schemas will be partitioned among types of database by whether the data must always be consistent(RDB's), are mostly readonly and can be updated leisurely (contact-lists, personnel records), and those that can afford to be inconsistent (archives, web chatter).
I see this as an area where Microsoft could pull out a big win. Imagine if, instead of their half-baked BigTable clone that they just released, they had instead put 1000 or so brains on the problem of distributing a single SQL Server database across N machines.
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.
Imagine if, instead of their half-baked BigTable clone that they just released, they had instead put 1000 or so brains on the problem of distributing a single SQL Server database across N machines.
Unfortunately brains don't scale, so you're better off using one huge brain, like Michael Stonebraker.
