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.
To sum up what everyone has said, AMD made the 955 a Black Edition for easy overclocking so they won't void it if you overclock within safe measures. For example, don't go over 60°C for your CPU and if you haven't hit that temp, don't go over 1.5V for your CPU voltage.
but since AMD would have no way of knowing you have overclocked, its only an issue if you are really pushing it to the limit and burning out the chip.
I see your point. Since AMD gives a warranty for the specs (the info listed outside the retail box), the warranty is only valid for those specs. SO anytime you're operating the chip outside of those, technically the warranty is void even though AMD has no way knowing what you're doing unless it comes to a situation wher you have to mail it back for warranty work.
So, welshmousep is correct. Technically, the warranty os always voided when you're outside the specs. However, without the proof (ie, a burnt out chip) there's no penalty. In the meantime, have the pleasure of OC'ing and enjoying the benefits of performance that you're getting for free.