Warranty void upon opening - I've never seen anything one way or the or the other on this. However, Apple does technically have the right to deny warranty claims based on physical damage caused by opening, regardless of whether it appears to affect functionality or not. Whether they will or not is probably up to the individual technician working on it.
They took my ram - Pretty much everything you leave in or on your computer is your responsibility to retrieve prior to service. Stickers, discs in drive, ram, data on the hard drive, etc... If you put it on it or in it, then you can take it back out.
They swapped my computer when all it needed was the sound board - I noticed today that Apple doesn't have the 2009 mini logic board listed anywhere on their site since it's so new. I suspect that anything currently diagnosed with a bad logic board will have the entire system replaced until they get this part into their inventory. I would probably replace any user installed items in the mini prior to taking it into service or risk possibly loosing them for good.