This is interesting. According to All Things D, only 5% of tablet buyers buy anything other than the WiFi version. It goes on to say that even the tablets sold with wireless chips often go unused, even when cellular enabled. Article and graph here.
What they fail to mention is that most smartphones have a mobile hotspot – or to mention how many people just assume they’re already paying for one data plan (hotspot on phone or MiFi) so why not just tether it that way?