Today, Samsung officially announced their Galaxy Tab, which comes with full 3G connectivity and the ability to make phone calls, just like a regular Android smartphone. Samsung says that in the future people will carry multiple SIM cards for multiple gadgets. That seems convenient for Samsung and other manufacturers doesn't it? However, I think it's a true prediction.
Carrying an extra SIM car is not that much of a big deal. Some people already have their SIM cards for phones, and they carry an extra USB pendrive that gives them 3G connectivity for their laptops. That pendrive already contains another SIM card. So is it that much of a stretch to consider paying for a data plan for your tablet, as well? I don't think so.
The carriers will profit even more from this than manufacturers. After trying to stop the ubiquity of data plans for so long (before iphone), while being afraid that data will kill their voice plans, carriers have finally realized that there is a lot of money to be made from data – even more than they ever made on voice. And opportunities like this is what's going to allow them to make more money from data plans.