There's a great talk by the CTO of Facebook available on Youtube, and I think he gave the right advice on scaling:
Don't worry much about it until just before it becomes a problem. Don't overengineer, because you're likely to waste time doing something that turns out to be irrelevant.
He is also talking about a website. The scalability issues with websites are well known and solved and can be easily tacked on as you get bigger. So yeah you don't need to worry about them when you start. bitcoin is a whole new beast.
Having a collection of servers that act as transaction clearing houses for people would probably work but it seems to break the whole p2p idea of the system. And I think isn't necessary if you design the system right in the first place.
It seems like the best thing would be to lay the system on top of a DHT like kademlia. Maybe you can do this later I'm not sure. Again the documentation is lacking so it is hard to tell if you can easily break the chain apart in order to stick it in a DHT later. I was hoping the devs would stop by and answer maybe they are too busy improving the docs...