Whoever said that replacing the HDD in the Macbook Pro yourself does not void the warranty automatically is correct. I read the Applecare agreement over a few times and it only states that it doesn't cover any damage caused by the installation. They may try to bully you if you need repair service, but ultimately they have to prove it was caused by installing your new HDD.
I asked someone at the Apple store if they would do it and they redirected me to a local Apple certified tech center. They wanted $100 just to swap the drive - no data transfer or OS install! It was a rip off so I did it myself and it turned out great. Well worth it!