I'm going to talk about "is" vs. "ought" but I'm going to use the words "description" and "prescription" because "ought" is an ugly word.
David Hume's immortal observation that prescriptive statements cannot be derived from purely descriptive statements is under attack by smarmy asshat Sam Harris, author of a philosophical abortion called The Moral Landscape.
I personally agree with David Hume, and while I don't have much time to write right this second, I want to briefly clarify what this important disjunction between description and prescription means to me.
In a deductive argument, there is no case in which a prescriptive conclusion can be derived from a collection of strictly descriptive premises. If a prescriptive conclusion is reached through a valid deductive argument, there will always be a prescriptive statement somewhere among the premises. Value judgments are always assumed, though in many arguments they are brought in quite cryptically. They tend to be (but are not necessarily) implicit in words like "good", "best", "should", "ought", and their opposites.
To give an example, one might argue that it's raining today, therefore Sally should take an umbrella if she goes out. This has the form of an argument in which a prescription has been derived from a prescription, but clearly there is an implicit premise that Sally should avoid getting wet, as well as a standard piece of normative logic that looks something like "(A should P) and (If A does Q then A will P) implies (A should Q)", a kind of moral modus ponens.
Oog, I gotta go.