Is there an existing capability to allow editing of only pre-existing pages? If not, a good way to implement this?

I’m working on a setup for which I need general users to be able to edit a specific list of pages, but not create any new ones.

Looking through the Codex, I don’t see anything that will come close to this desired functionality, but I thought I might be looking through it.

Failing a pre-set, I started looking into Advanced Access Manager, but it was not immediately obvious to me if it would do what I need. Anyone with experience with this plugin have any idea where to begin?

In WordPress 3.5 there will be a new capability create_posts. You could create a new user role or change the role editor and remove that capability for this role.