Not sure if anyone else has found this. On my mac I had been using the -useoldlauncher till today and it was working. Now all I get when that is used is a black square on the screen and nothing happens. It seems that it has stopped working now.
Having changed the -useoldlauncher to -dx9single it has now worked and the new launcher is running fine. Still to confirm if all this works for logging into the stress test tonight.
off topic: Shame really as the old login screen with the moving background was far nicer than the current one