Years ago when the iPad first came out, I believe you could do this. But it didn't last long. I remember pulling my sim card from my iPhone 4 and trying to use it in my iPad. I forget the details, but it did require me to get a separate data plan (it wasn't all that clear) and long story short I had to spend some time with ATT and explain I didn't really want a data plan on my iPad.
I don't think you'll be able to do it - at least not with an unlimited plan.