Just one additional point- in my experience, and i have tinkered with my plan to find this out, they do seem to include the texting plan in whether an individual line is over $100 for purposes of a full upgrade. Like my current plan now is the 900 minute $59 voice, $30 unlimited data, and $20 unlimited texting, for my base before taxes and additional features of $109. This is a single individual line, and my upgrade screen shows that I will be eligible for a full discounted iphone upgrade on June 24, 2011.
And like I said, I tinkered with my plan several times to find out what would change the date and what wouldn't. Like for instance adding the navigation app for $9.99 per month doesn't do it. Adding other features like the car towing service doesn't. When I had the 450 minute $39 voice plan, that wasn't quite enough. It has to be OVER $100 per month in voice, data, and texting plans on an individual line.