I can run it like this from terminal:

cd ~/Qt5.0.0beta1/Desktop/Qt/5.0.0-beta1/gcc_64/bin
./QMLViewer.app/Contents/MacOS/QMLViewer ~/Desktop/rect.qml

but there don't seem to be any other use cases that work.

1) if a qml file is not given, the viewer just exits. Where is the old animated splash demo thing which shows by default in Qt 4.8?

2) qmlviewer.app invites the user to launch it like a typical mac app, it doesn't look like it should just be a command-line tool. So it should support right-clicking on a qml file and using "Open with...", and it should support dragging qml files onto it. And if it didn't exit on startup, you could at least open a qml file from the menu.