Back up your MythTV database before upgrading!
It saved my ass today. I started the new frontend, called by KDE's greeter out of init, and it choked on the database schema upgrade. This left me with a half-completed schema upgrade, which made both the front and backend choke hard, as well as the mythtv-setup program. I had to drop the database, recreate it, restore from the backup, and run the backend all by itself to fix things.
The old problem of, first execution after reboot, the wrong card on the disconnected tuner being the only perceived recording device has gone away. It has been replaced by the new problem of the second card not being visible to the recording scheduler. A kill-and-restart solves the problem. I've successfully recorded simultaneous programs on ESPN and ESPN2, which clears the way for the next two weeks.
I also got MythWeb running. It's way past fucking cool. I can finally set up recordings when I'm away. It looks like there's some nicer, more powerful search and auto-record features now, akin to TiVo's Wishlists. This upgrade wasn't as painful as I thought it was going to be, but you never know with rev <0.50 software.
Next up is to compile the frontend for OS X, so I can watch recordings from upstairs...