Within the url http://msdn.microsoft.com/en-us/library/ms189053.aspx#Restrictions
at the section titled "Restrictions",
the comment states "the principal and mirror server instances must be running on the same version and edition of SQL Server".
This statement is misleading at best, since Mirroring to a lower version of SQLServer to an upper version of SQL Server is possible, certainly if the lower versions are the correct upgrade path service packs. I have not tested inter-edition upstream mirrors yet, but I see no reason why that would not work either (especially assuming that the lower stream database does not use higher edition level functionality).
So to summarize, . You CAN Mirror a database from SQL Server 2005/ 2008/ R2 -> 2012. It is known as an "upstream mirror". You should ensure that your service packs are correct for the lower version 2005 =>SP4, 2008=>SP2 and 2008R2=>SP1. Also use SSMS 2012 if you are doing a GUI based mirror setup.
Upon fail-over to the upstream higher version the mirror will be broken and mirroring will be suspended. It is not possible to reverse mirroring to a downstream version.