> This is stumpifying!
I agree. But I think we're getting closer to pinpointing the cause.
Your response to Ipstenu's question suggests the issue is specific to your copy of the TwentyTwelve theme. Otherwise the Vanilla Twenty Twelve test would have worked.
If you have access to a tool that lets you compare two directories (e.g. the diff command), I'd suggest downloading a fresh copy of the TwentyTwelve theme and trying to isolate the difference.