Selecting different cameras will certainly be made easier in the future, although this will not be included in the next update I'm afraid.
For now the viewport follows the camera associated with the last selected/viewed Render node. You may have to press the View button to make sure you're viewing the current camera though. The simple process to switch (although admittedly it is a kludge) would be to click a render node to bring up its settings, either in the Node List under Renderers, or in the Network View, then click View under the 3D Preview Window.
- Oshyan