Dave, make sure you are following the processes as described in this url

http://support.microsoft.com/kb/842425

you will have to make some changes (db restore not required as you are mirroring instead) but make sure the gotchas it refers to are covered. You may have two installation ids and have to delete the one from the primary.

also be sure configuration via reporting services configuration manager is identical on both boxes (except for the connection server in the database settings tab.)

You would have to an initial reporting services install on the standby server and then drop the databases and start mirroring from primary