I think this is what you are looking for. I have one and it's awesome! It has a small, half moon shaped, super sharp razor blade that comes out of the end, and you can "unsew" a seam in no time flat. They also have a rotary cutter that is just as good (Amazon also carries it). Anyway, here's the link:
http://www.amazon.com/Gingher-3779-S...ds=seam+ripper