Hello,
I believe that message is just letting you know that your transitioning from Mirror to Principal role. What happens there is that your Mirror will roll forward the database, how long this will take depends on the DB activity and size of your redo queue, and then make it new Principal.
That's a bit simplified explanation though, I remember seeing MSDN article explaining all the phases in failover transition in more detail.
Mika Sutinen, Senior DBA
@SQLFinn on Twitter
Blogging at SQLStarters[/url]