I think the majority do, yes. Personally, I don't even have a cell phone. As there's not enough cell coverage at my house to make a voice call, I think 3G service is still some ways off for me.

I have WiFi at home and WiFi at work, and I'm rarely anywhere else. At least, I'm rarely anywhere else I need Internet access.

Nokia has said they could add a SIM slot to future tablets, but they've indicated they have no interest in doing so. That could change, of course.

EDIT: Nokia seems to be using the Internet tablets as a sort of research project. They're not keen on having cellular providers (particularly the American ones) standing in the way of getting products into the hands of consumers or limiting the ways consumers get to use those products. At the same time, Nokia seems to anticipate that the telecommunications transition from "data over voice" to "voice over data" will reach as far as the general end-user in the not-to-distant future.

Nokia seems to be positioning itself so that when WiMAX or a similar technology takes off, they have a product ready to go, complete with good 3rd party support.