The question "what problems will I have when migrating to the cloud" gets asked often enough. If by cloud you mean Amazon EC2, then from a technical perspective there isn't much that changes. The biggest thing that changes is just how you pay your bill.
Having said that, there's still a few potential gotchas:
work on the principal of having a floating IP address which is used for the application to connect to the current master. With EC2, you can't do this.