This is a myth created by internet service providers.
I've heard loads of boogeyman stories about people getting a surprise astronomical bill in the mail without providing proof.
I'm calling bullshit.
I've tethered for nearly 3 years without hearing a peep from Verizon.
I think that could be true -- the huge bill being a myth. For one, there may or may not be caps on the usage when you're on the "unlimited" plan. But if there's an overage charge, they would have to tell you up front.
Or perhaps, these apply to the limited plans. It used to be the case that Verizon allowed tiers of use for smartphones. When I had my Treo, over a year ago, you could get some fixed number of MB, or "unlimited". If you went over on the fixed plan, you did have a per-MB fee to pay.
But getting kicked off for too much use.. that definitely happens, with Verizon, even on "unlimited" plans... or has in the past. I used to have a mobile broadband account with Verizon. That's not tethering, it's direct to a laptop or other computing device. Today, they put a 5GB cap on it, but some years ago, it was written up as "unlimited". And no, they couldn't charge you for overages, but the really did go after the top "bandwidth hogs" and kick them off the system... some of the folks I knew online had this happen.
There seems to be a belief at Verizon, AT&T, and the others that 5GB is, in fact, "unlimited" if you're using a smart phone. This is clearly untrue (I could easily exceed this limit on my DROID by streaming YouTube videos for a day or two, or about 80-100 hours of streaming music at high quality), but that seems to be their current notion. So if you exceed that, they are going to believe you're tethering or doing something else forbidden (it's actually in the contract), and based on that, they can kick you off. I don't believe they can charge you extra.