The experimental API v3 does not support polylines yet. We don't need polyline support for drawing simple rectangles. We can make browser to draw them.
BoundsBox for v3 is an extension that takes LatLngBounds object as an argument and draws the bounds on map. A DIV is created and made an OverlayView overlay object.
Styling of the rectangle is made with CSS. You can put text content also and even a background image. 'click' event is available, 'mouseover' is easily attached.
See the example that visualizes bounds that v3 geocoder returns.