During Kedusha, one is to stand with the proper physical attitude (or, if one is still davening, stop and listen, at least according to some opinions). But when does Kedusha begin? Knowing when it actually "starts" would affect someone who is walking in, or someone who is still davening his own Amida and has to know when to pause.
Does it begin when the chazzan finishes the bracha of mechayei hameitim? Does it begin when the Chazzan (after waiting for the kahal to say the opening words) begins saying "Nekadesh"/"Na'aritz'cha"/"Na'aritzach"? On the Yamim Nora'im, is it when we get to "Keter yitnu"? I have even heard that it only really starts when someone says "Kadosh, kadosh, kadosh" -- but I don't know if that refers to the Kahal or the Chazzan.