Hazza. Was in your situations a few months back. Discolouring is a common problem, and Apple SHOULD replace your case, along with any cracked parts. Mine was 6 months old, and looked 5 years old. Apple requested I sent it to their authorised repairer, who broke promise after promise of when I was supposed to get it back. A month later, it arrived, and looked like this;
The repairer never bothered replacing the topcase, merely made a pitiful attempt to clean all four corners (PIC 1). Never bothered replacing the batter cover either, so I had a nice pristine white bottom case and a lovely yellow square (PIC 2). They never put the keyboard top cover back on properly, and was buldging all the way around the MacBook (PIC 3). Cross-threaded a screw in the battery compartment bay (God knows what the insides looked like) (PIC 4) and managed to chip away at the topcase that they never bothered replacing (PIC 5).
To cut a long story short, Apple requested I send them these pictures and were confused initially as to why the top case had never been changed, as it was covered under, and even outwith warranty. They called an hour after receiving the pictures above and decided it was in their best interest to replace the MacBook with a brand new top of the range (at the time) version of the 13" Unibody MacBook (which was worth around £400 more).
Bearing in mind this took a grand total of 2 months, and 3 weeks in I bought a 17" unibody instead. Sold the little one to a friend for £900
My advice to you is to phone Apple as ask to speak to customer relations. Be calm, but emphasise how disspointed and upset you are. Again, be calm. See where you go from there.
edit: The cracking along the palm area is caused by the nibs along the top of the screen. Over time (with closing the lid), it weakens the plastic. The discolouring is caused by cheap s**t plastic and the heat trying to escape from corners and crevices. Have a look around the infrared port and any other bits like that, and I can guarantee you'll she lots of tiny cracks. Again, shitty plastic.