There seems to be 2 stances given by Apple, as I said previously and Macworld have said, doing it yourself voids the warranty and requires some disassembly. Now that we've seen the internals, the disassembly part is evidently true.
However, I am certain that the official stance now is that it won't void your warranty unless damage is caused but as AppleInsider have said, you will need to reinstall the original RAM before Apple is willing to work on the machine.
Again, in the USA the FTC has been warning companies all year that they can't void warranties for customers opening electronics and/or repairing and/or using "unauthorized" parts.
Apple was on the receiving end of some of these warnings.