I don't think it will be a big problem. While some will use the Tab (or other tablets) as portables on the road, I think most will use them around the home/office. I think there is a significant group that won't want to pay for another data plan, and would rather buy a cheaper (wifi only) version. If you need data on the go, you can always tether to your phone.

Here's how I would market it (with a couple of the right apps):

You come in the door of your home, take out your android phone, tap a shortcut on the screen, then plug it in by the door. You walk into the living room and grab your Tab (or other tablet). Obviously, your email and contacts are in sync. But, while walking to the living room, your tab connected via bluetooth to act as a speakerphone/handset for your phone by the door. That interesting web article you were reading on your phone is open on the Tab. You finish that, and switch over to the ebook you were reading at lunch (or tech manual at work). The Tab opens up to the same spot you were on your phone. That new high score you got on your favorite game? It's right there on the Tab.

In the morning, you eat and read some sports news, not finished, you hit that same shortcut on the Tab homescreen, put it down, grab your phone and head out the door. In the train/cab/bus/etc you pull out your phone, and finish reading the news from where you stopped at breakfast.

In short your wifi-only Tab become an extension of your phone. One app to sync certain information to make a seemless transition from one to the other. Of course, this would work equally well on a 3G Tab.

That would make a nice little TV commercial, and I think a lot of people would love it. Of course, you still have to work out the price points. Make it an included Samsung app on their android phones and the Tab, and perhaps get multiple sales to the same person. At least until someone copies it or does it better.