I buy the concept of non-exclusive carrier choice for the iSlate (or iWhatever).
What I don't buy - literally - is the likelihood that people are going to pony up for yet another expensive data plan. I have internet service at home. I have internet service for my iPhone. I have a 3G dongle for my MBP when I'm on the road. That's it. I'm not adding a fourth internet contract/fee/expense. No way.
There are a few ways Apple could handle this. And maybe if the price is truly approaching $1000, maybe they've figured out a way to bake 3G connectivity into the purchase price (as has Amazon). Otherwise, I hope they sell a version that either doesn't require 3G or one that will finally take advantage of my iPhone's ability to tether.