Switching baselayers as user zooms in to map in OpenLayers?

1. Add 2 base layers to the map
2. Subscribe to the map's zoomend event
   • map.events.register(type, obj, listener);
   • http://dev.openlayers.org/docs/files/OpenLayers/Map-js.html#OpenLayers.Map.EVENT_TYPES
3. Check the zoom level and just switch the base map for your user
   • map.setBaseLayer( layerRef )
   • http://dev.openlayers.org/docs/files/OpenLayers/Map-js.html#OpenLayers.Map.setBaseLayer

Here's a sample page with an event handler for zoomend - http://www.openlayers.org/dev/examples/events.html.