You only live once.
I might call and ask for a credit (that's a good idea! worst that can happen is they say no), but I'm not going to quibble over it since they are already discounting it somewhat ahead of their contractual obligation.
I could wait it out, too. The 4S is perfectly fine (unlike previous phones where I really needed larger capacity and/or faster CPU), but if I wait until my proper upgrade time, I'll be too late for even early upgrade pricing on the 5S (or whatever).
So, c'est la vie. You want to keep up on the iPhone, you gotta pay your dues. Besides, selling the iPhone should recoup a large portion of it.