MMS is not out for the iPhone 3G S. trust me. i have one.
(
"MMS is not going to be released until the end of summer" says AT&T and Apple)
if you are going to get the iPhone, then here is what you will expect to see on your first bill
Bill one : $300 or more. mine came out to $346.89
(Activation Fee= $36.99)
but the thing is that i went over it with my mom, and she did the stats of the bill, and its really 2 bills in one. so you are actually seeing 2 months on every bill. Thats a way of going green

now on the next bill it will be in the mid $100 range. mine will come out to $159.78 with the 1 month rate of: $79.89
it really depend on what plan and features you get.