I'm sure everyone knows this already, but in case it was never clearly stated...
I went in for an upgrade (Launch day TBolt here) last weekend with a sliver of hope that because the account holder on our family plan still had unlimited data, I MIGHT be able to piggy back off of that and keep unlimited for my line also.
No can do. They told me if I wanted to upgrade my phone at contract price, I had to pay full retail. I told them I was switching that day (free GS3 from T-Mobile? Yes please!) if that was the case, and he didn't change his answer.
So the absolute only way to keep unlimited data AND use your upgrade at this point in time is through the loophole discussed in this thread. No piggy-backing, so to speak, off of other lines' contracts at all.