Scaling out SQL Server and syncing data across multiple machines
Creating a Scale Out database at Internet scale is pretty huge step. You will face a lot of issues that are not critical on a single big database. From your notes I see that you understand some of the basic issues you face. 

Since Microsoft has papers on using SQL Server for scale out, I suggest that you study those first. Your scale out strategy will need to take into account the database server you choose. 

For Microsoft SQL Server you should first study: http://msdn.microsoft.com/en-us/library/aa479364.aspx

This paper discusses the decisions that you need to make and why they are important. It offers 5 SQL Server strategies for scaleout:


  • Scalable Shared Databases
  
  • Peer-to-Peer Replication
  
  • Linked Servers
  
  • Distributed Partitioned Views
  
  • Data-Dependent Routing


As you go down the stack, things get more complicated, but also provide more powerful ways of scaling out.