It's only voided if AMD has to make a determination. So as long you're overclocking and your CPU is stable without excessive heat, then the warranty issue doesn't come into play.
However, if your CPU burns up dues to excessive heat and you try to RMA under warranty. AMD will thoroughly inspect it (microscope, magnifying glass, etc) for physical damage (burn or scorch marks, bent pins, scratches, damaged edges from drops of rough handling, etc) and if they determine you caused the damage through due to negligence or abuse, they probably WILL VOID your warranty.
Overclocking in itself does not void the warranty. For example, a tiny overclock like 5 percent shouldn't kill a chip but if you get too overzealous and go for something like a 40% overclock and 1.7vcore (crazy!) you'll probably burn up your chip and maybe the mobo too if you can get it to post and run at that extreme overclock.