Microsoft SQLCAT team members Sanjay Mishra and Prem Mehra have published a new Technical Note called Mirroring a Large Number of Databases in a Single SQL Server Instance on the SQLCAT web site. This paper explores what happens to performance as you mirror more and more user databases on a single x64 instance of SQL Server. They wrote this paper to address a common misconception that you cannot mirror more than 5-10 databases on a single instance of SQL Server.
According to their benchmarks, performance is not really an issue (on x64). What I have seen in my own experience is that the administrative overhead of having high numbers of user databases (that may be dependent on each other) is the real challenge. I was one of the technical reviewers for this paper, which was a lot of fun.