My MBP manual has instructions for the user to swap/upgrade both the HDD and RAM. This isn't a problems and will not void the warranty. The battery, however, is clearly marked not to be replaced by the user and doing so will void the warranty.
I'm sure I'm capable of doing iy myself, should it be available for sale through 3rd parties. Problem is, I don't want to void the warranty. Of course, this may not be an issue in the end, since the battery is capable of outlasting both the 1 year standard and the 3 year Applecare, but that depends on how we use it. Chances are only the 3 year Applecare may be at risk. Of course, without instructions from Apple, we also don't know if there's anything at all that we're supposed to do for a battery replacement other than the physical replacement itself.
Also, the MBP knows some component P/Ns such as the HDD as soon as we install/replace/upgrade it. Maybe there's circuitry in the battery that communicates this info back to OS X? If OS X records this, anyone at Apple would know and void your warranty. Food for thought.
In any case, I was just wondering why Apple mandates their own battery replacements. There must be a reason, and this we want to know before we start doing our own, if that makes us feel better. Think about it, if Apple trusts us to replace plug and play items like the HDD and RAM...is the battery a 100% plug and play item? Why not let us do that as well?