I'm not a lawyer. I'm not your lawyer. I'm not StackExchange's lawyer. That said, so long as there is even one legitimate product that someone could feasibly purchase without violating state or federal law anywhere on the entirety of Silk Road (which there is) then it's possible for someone to be asking a perfectly legitimate question with no legal implications. Unlikely, sure, but possible. This limits our liability and should always be the guideline when deciding if a question or topic should be closed/banned.
Similarly, recall that while Bitcoin is a massive multi-national distributed network, StackExchange is not. Their servers, offices and staff are located in New York and this modifies a number of legalities. See this question about growing marijuana over on gardening.SE, for example.
Given the precedent set by other sites I would say that Silk Road questions would be both allowable as long as they do not contain explicit references to illegal activity (i.e. "How much BTC should an ounce of heroin cost"). If you would consider the question on-topic if it were about, say, BitcoinStore or Blockchain.info then it should be on-topic for Silk Road as well.
Of course this could be tainted by my own personal viewpoints on the subject, so feel free to vote your conscience.