How do I rotate the ipad canvas in Xcode 4's Interface Builder to landscape orientation

You can rotate controllers, not Windows/Views.

To do so, select the view in the editor, then use the 'Attributes Inspector' tool/view (4th icon out of the group of 6 along the top) on the right - or use the menu.

Set 'Simulated Metrics/Orientation' to the orientation you wish to use in the editor.

If you select an xib on the left, then you can select different components to edit. If editing focus is on a 'window' or 'view', the orientation option will be dimmed on the right. A dimmed option indicates you need to select the controller instead before you make changes.