1). As long you're overclocking and your CPU is stable without excessive heat, then the warranty issue doesn't come into play.
2). However, if your CPU burns up dues to excessive heat and you try to RMA under warranty. Intel/AMD will thoroughly inspect it for physical damage (like burnt prcessors, Bent Pins etc) and if they determine you caused the damage through due to negligence or abuse, then probably WILL VOID your warranty.
3). Overclocking in itself does not void the warranty as long as its stable & working as specified.