When zooming with rulers on, the ruler does not update properly until you scroll.
When going from zoomed in state to zoomed out state, if entire image is visible, ruler does not update and is not scrollable to update.
I have a Snow Leopard Mac and I have the same problem, Photoshop CS5.1 does not update the rulers when I zoom using the zoom tool or Command + / -. However when I use Command 0 it resets them. Also if I scroll around in the document they reset the instant I start scrolling.