The 90 days could be a problem. If I upgrade 2 lines / trade in 2 phones (both iPhone 4) I will get $400. The service portions of my bill for 3 lines on the 10GB Mobile Share = $121 each month. So 3 months = $363. I suppose I can use the other $37 for a case for the new phone.
Two lines with 10GB should be $130/mo, no? $100 for the 10GB MSVP + $15/line. That would put you at $390. Still missing 10 bucks, but at least it's closer.