I must admit that a couple of times playing Rage I noticed a bit of stutering, despite 99% of my playtime being at a constant 60 fps.
I suspected something like this, as it's the same that happens in Unreal Tournament 3 if you set it to low priority for the cpu - but I have no idea why they should set it to something like this.

It basically means that if something as simple as a background application requires some slight computations at times, the CPU will rather take care of that first than the game - which means there will be 'holes' where the cpu isn't processing the game and then you notice stuttering/frameskips. This severely hurts users with fewer CPU cores than 4 (triple/dual/single).
It's particularly bad in the case of Rage, as it constantly streams textures in and out of memory from a heavy/demaning format and recompresses them to a format the GPU can use all the time - nVidia users have the luxury of the "GPU Transcode" which moves the process to the video card.

Adding insult to injury, if the set the CPU priority for "Rage.exe" in the task manager to something more reasonable, like "Above normal", the game automatically forces it back to Low.