The way I look at it Apple designed this system to be user serviceable. Even if they don't want you changing the CPU, they can't invalidate the warranty for replacing the CPU yourself unless you caused other damage, according to the Magnuson–Moss Warranty Act. I suppose they could be nasty and argue that you installed a CPU that wasn't supported (since they don't offer that model) and that caused damage to the machine. So to be safe, as far as the warranty is concerned, I would just install the original CPU before sending it in for warranty service. Same for RAM or any other component.