One of the scrum teams we have in our company is called the "System" team. They are the closest thing we have to a QA department now.
They take the work from all the other scrum teams working on the product and merge them together to do System testing and resolve any performance and integration issues that may arise. The team has a heavy focus on testing at a higher level, but also does some development (mainly tweaks and System-level fixes).
This scrum team essentially works a sprint or two behind the other teams. Some of you may say this breaks the idea of Sprints - the work produced in a sprint should be considered 100% finished, testing and all. First of all, that's the idea in theory but in practice things don't always work out that way. Second, each team is only focusing on their part of the product. They can't possibly be expected to verify that it integrates well with all the other components of the system. So we felt this higher level role should be satisfied by a separate scrum team.