i found this video on youtube. i'm guessing a worker or someone was taping this secretly while MJ was rehearsing. & indeed, i do hear him rehearsing Dirty Diana. you will need to turn your speaker up LOUD in order to hear.
[youtube:2br5hcr5]http://www.youtube.com/watch?v=-16TxpaKmqg[/youtube:2br5hcr5]
you can hear clearly hear MJ say "Dirty Diana" around 0:59.