How does putting everything into a playlist and emailing it differ from just emailing a backup? The idea is that you delete all music from the slave device and then import the backup from the master.

I also consider my iPad to be my master file and I sync that to my iPhone. What I have been doing, though, is syncing the iPad to my Mac the usual way, and while it is connected I copy the file UserSongs.plist to the desktop of the Mac. Then when I sync my iPhone, I replace its existing UserSongs.plist with the one from the iPad. I was having problems syncing last week so I'm not 100% sure this still works in the new version, but I'm pretty sure this is what did eventually work rather than the email method.

Emailing is problematic for me because the Mac grabs all of my email and deletes it from the server. So to email to my phone I have to shut down Mail on the Mac, email the backup from the iPad, grab it on the iPhone, restart mail on the Mac, clean up the chaos of the emails that are now on the wrong devices, etc. Copying the file is far simpler.