Just in case you have missed it. The SQL CAT team have written up a great article that answers the question on just how many databases can you mirror within an instance. This helps to debunk some of the figures that were documented in SQL Server BOL.
Its a must read for anyone planning a consolidated SQL Server environment with the use of database mirroring for DR.
The blog post is located at - http://sqlcat.com/technicalnotes/archive/2010/02/10/mirroring-a-large-number-of-databases-in-a-single-sql-server-instance.aspx