your macbook must be significantly older than a year if you were out of warranty when the crack happened, you got it repaired and it now has another crack? I doubt that all happened within a few weeks of your applecare running out? If it did, raise hell with Ken Bell, he is Steve's big CR guy. I'm sure he could do SOMEthing for you...
My first crack developed approximately 8 months after I for recieved my new macbook. Warrenty however was refused because i had previously replaced my lcd (having it done by apple would have been in the 800 dollar range). Anyway hoping that apple had taken moves to correct the weaknesses in the keyboard top case, I bought a brand new one from apple. I've been using it for just over 3 weeks now and yesterday while cleaning it I found a new crack starting very near to where the last one was.
In regards to some other users comments, I can take a picture of the crack if you'd like but it is virtually identical to the cracks that I have seen on other macbooks posted on the internet.
I am not rough on my equipment and in fact I treat everything very well, with a great deal of respect. I have grown up and spent years working with expensive equiptment and know how it is ment to be treated. With that said I am probably not as easy on my computer as some people here on. It does travel with me daily (in a very good protective case) and it is used a great portion out of every day. I am well aware that some people's computer is treated like a prized jewel and spends all of it's time short of being wrapped in goss, perhaps that's what it takes to make the computer last up, but that kind of defeats the point of having a lap top don't you think?