HP/Compaq. I know for sure, gateway and emachine doesn't, you open and its over. As for sony, i dk. Dell, i believe you can, but if you break something intentionally, it's not their problem, though depending on the state you're in.
ROger
In the past, I have managed to add a DVD drive and a second hard drive to a computer and replaced a sound card, although it never went smoothly. I feel fairly confident that I can manage to upgrade memory.
I don't feel confident if my need/problem extends to replacing a motherboard, power supply, or several other parts that could go bad. Having the first year under warranty is a nice security blanket and I might be persuaded to get an extended service plan if cheap.
I really do need to replace my desktop and I would like to consider a system which I can upgrade later. It would give me a bit of financial flexibility and probably be more fun if I could upgrade later, plus I might have more interesting options. For example, in the short run, I don't need more than 512MB RAM, but I foresee the potential need for more in about 6 months if things work out. If it were cheaper to just upgrade the RAM when I need it, I would do that, but not when I need if it invalidates a warranty.
So, which computer manufacturers or retailers do not automatically invalidate a warranty or service plan if you open up the case for any reason?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic