I applied to a machine that was definitely broken, logged out and logged
back in and things work with that added argument.
If you're still doing stuff in, say, an hour from now, go for a sync and update the system at that time. Remove the -rrxinerama workaround and create a new session without it.
I've pushed a fix for the wrong screen size with RRXinerama. Packages are currently building on launchpad. Upload ETA ~30 minutes.
Mihai