Hastings replied to that question in the Netflix blog post's comments: [1]

Question: "If a film I search for on Netflix is not available for streaming, will the website still tell me if the DVD is available? Or must I search twice?"

Reply: "ouch. You'd have to search the second place if we didn't have it in the first place."

[1] http://blog.netflix.com/2011/09/explanation-and-some-reflect...