Good! Glad to hear I wasn't crazy about this having been the old behavior. I would be fine with Ctrl-up and Ctrl-down. Or if this action would be configurable via key bindings, I could just set the bindings to override the multiline behavior.
Actually, I would think that multi-line should be an option that you would enable/disable via another icon-button on the left, since it would seldom be used. So if you are in single-line mode, just stick to up and down arrows.