from what i know. you can't detach a 2008 database and reattach it to a 2012 instance.
Can someone confirm that you can only attach and detach between the same major version of sql server.
Also... you can backup and restore to a higher version of sql server.
ie.... backup a db on a 2008 sql server and restore it on 2012 sql server.
you can not go backwards in backup restore tho
ie backup 2012 db and restore to 2008