I'm surprised that is news to this audience. The head of NFL mobile was on a pod case (this week in Venture capital) and he had great statistics about traffic to NFL.com.
IMO NFL.com is a great proxy for a mainstream US sports site. He said about 60% of their traffic was mobile, 10% of mobile was tablet. This was interesting to me: mobile traffic was about 50% their app and 50% mobile web. That stuck in my head: apps are growing, but the mobile web is growing with it.