There’s definitely something fishy going on with motion blur in 2.71 for Windows. I’m using 2.71 x64 “zipped” and with motion blur enabled Blender doesn’t render the cubes at all:
2.71 x64 zipped
2.70a x64 zipped
As soon as you turn off motion blur in 2.71 the cubes reappear.
Something similar seems to happen in your case: the cubes are partly transparent - you can see the edge of the ground plane shining through in the upper half.
I’d say: Bug it.
I just tried the latest Blender buildbot version from today and get those weird half transparent cubes seen in your render. So, obviously there already has been some bugfixing (2.71 “official”: no cubes / 2.71 buildbot: transparent cubes) but the result is still unsatisfactory…