I've never asked about LinkedIn in an interview. However, I do sometimes look up candidates prior to interviewing them, and admit to being slightly curious if I can't find them, although would not let it count against them. Conversely, I have also seen a few candidates look me up on LinkedIn prior to their interview, which I generally take as positive sign, because it shows they are trying to prepare.
For quite some time I was trying to get people to look at my actual resume instead of its shadow on linkedin and I couldn't get people to do it because they would google my name and the first link would be on linkedin so they would never bother to look beyond it. As soon as I closed my linkedin account all of a sudden people started looking at my actual resume and my code on github. I don't regret closing my account and forcing people to do some legwork to find my qualifications instead of doing the lazy thing and stalking on linkedin.
