This is present since at least v5 and still in the latest v6 sr13:
When the perspective view is set to parallel and default orbit rotation (world axes) it locks up when the view is set to exactly align with the z-axis (viewing from top)
Once this lock happens the right mouse button starts to act as view-pan command instead of rotation.
This can happen quite often as the view rotation always happens in fixed steps.
And related: I guess the view orbiting is hardcoded in the deepest depths of rhino, but it could benefit from some attention - the orbiting resolution defaults were fine for the past, but with high-res input devices it looks and feels a bit to choppy… this is very noticeable in ortho views, perspective feels fine most of the time… maybe a new setting for v7?