Has anyone read (or written) anything on Scaling the new in-context UI (SiteEdit replacement)? Looking for guidance on how many servers (and how to configure them) for say > 100 concurrent authors/reviewers.
Experience Manager runs on top of the CME (Content Manager Explorer), so you can scale it out by scaling out the CME.
I would recommend making a baseline measurement of how many concurrent users you can cope with on a single box (making sure you measure DB impact), and scale out from there.
As Nuno says you need to scale the CME servers and as a resultscale your XPM infrastructure.
I have done this in the past, with SiteEdit 2009, and found DB connectivity to be the limiting factor once you reach a large number of concurrent users.
Scaling can only be as good as your smallest bottleneck, especially with real-time CRUD operations.
You must be careful while planning to scale XPM. One important thing you must consider is the Session management. For instance if your Web Site is .Net and you are putting your session data in a different location for security and performance (Server State, SQL Server State or Custom State) it won't work since the ClaimStore (this one will contain your session preview token) is not serializable.
In this case you should use Sticky Sessions in combination with ASP .Net In Proc session state.