I've never heard of opening the computer case to terminate a warranty. In fact, even swapping the cards don't terminate it. All these operations are regular maintenance and expected events in the lifetime of a computer. Of course if you botch up and break something, fixing that won't be under warranty.
Pretty common stuff really, when I supported Gateway PC's they had a sticker over the edge of the case to stop you from gaining entry "void warranty" liked that stopped anyone.
PCD (local brand) machines same thing, also just take a look at the bottom of the A500/A1200 etc... (thats if the sticker is still there) it's been done for awhile now :-)
