I think that's not quite right. If you have a family plan at more than $100/month, they gave the early upgrade to one phone on the plan, but it didn't have to be the primary line. My husband is the primary on our account, but he had a 3G from 2 years ago so he was already eligible to upgrade. I am a secondary line, bought the 3GS at launch last year, and was originally eligible to upgrade in November, but I got bumped to now. My "individual line" doesn't cost $100, but it was the only one that wasn't already eligible. So I'm pretty sure it's one line per account that's over $100, no matter which line on the family plan it is.
Nah, the upgrade is applied directly to the primary line. It isn't a "any phone on the plan can upgrade" type of thing.