Querying my brain for specific sources turned up NULL, so I spent a couple of minutes on pubmed. It seems my statement was too confident.

There was a large metastudy which found some effect in a high quality subset of studies: http://www.ncbi.nlm.nih.gov/pubmed/19826127 And a commentary on it which says their definition of 'high quality' is bullshit, amongst other things: http://jco.ascopubs.org/content/28/7/e121.long

I find the criticisms in the commentary convincing, but I'd still rate their being a reasonable chance of an actual association existing, and therefore a non-negligible risk of an actual causal relationship as opposed to just some confound. I invite anyone who likes this sort of thing to give it a little more time.