my understanding is that as long as you install third-party RAM produced to apple's specifications, and don't try to put in something that shouldn't be in there, RAM installation won't void your apple care access... it just means that your RAM won't be covered by apple care. in my view, if i was having an issue with a computer, i wouldn't want to change the configuration to get the problem fixed and then change it back to how it was before the problem.
also, if the problem is that there third party RAM. OWC's memory comes with a lifetime warranty and their excellent no questions asked RMA process that I've used in the past.
edit: by the way, if you choose not to send in your apple RAM, transintl's price is $10 lower than OWC's for 6x2gb sticks, and it also comes with a lifetime warranty (but i don't know if it's advance replacement like OWC or if you have to deal with added lag time of sending it in. moot i guess if you're hanging on to the old RAM anyway)