The iPhone for Life plan is actually a lease. Technically you are paying $70 a month. $50 for the service plus a $20 a month lease payment.
At the end of the lease you can either return the phone or pay $200 if you want to keep it. If you decide you want to just by the phone I believe there is a fee involved, plus all the lease payments that you have remaining to pay. Also, I believe you can switch to Easy Pay from the iPhone for Life plan but doing that accrues something like a $35 additional monthly payment.
I've heard that Easy Pay will also apply. Easy Pay allows you to put a certain required amount down and then you make monthly payments. You can pay the rest of the phone off however you like whenever you like. Easy Pay lets you get a new phone each year as long as you pay the phone off. The iPhone for Life plan is two years.
Last but not least, if you do take the iPhone for Life plan, understand that if you ever switch to a non-Apple phone you are going to lose the $50 service and it will go up to it's regular $60 cost. The deal is only for the iPhone and not any other phone.