Actually, I don't think this is quite clear yet.
Originally Posted by HardRock
Notice how the player's question asked about SK specifically, and the blue response referred to him as a "boss":
The SK only drops yellow on your first kill of him per character.
That is incorrect.
The possibility of a boss (or anything, really) dropping magic gear or rare gear when you kill him remains intact on kills subsequent to your first. The probability may change, but it does not change to a probibility of zero.
To be clear, I am not arguing for or against SK being the Act 1 end boss. I'm in kind of a neutral stance on this at the moment.
It is also possible that the blue is referring to SK as a boss but not an end-boss. There may be such things as mid-act bosses or some such.