the trackpad is on the same plate as they keyboard, so it's not as easy as pulling it out and replacing it. If its stained and within waranty, AppleCare should do it for you.
Before you send it off, get a piece of paper towling, dampen it a bit (not so its dripping), and try to get the stain off. It might just come off easily
I believe there is a known discoloration issue. I assume that would only apply if you've attempted to clean it yourself and it cannot be removed. I've read of Apple replacing cases in such instances.
The only way to know for sure is to call Applecare or bring it in to an Apple Store.