Nullable columns can be in 1NF through 5NF, but not in 6NF according to what I've read.
Only if you know better than Chris Date "what first normal form really means". If x and y are both nullable, and indeed in some row x and y are both null, then WHERE x=y does not yield true. This proves beyond reasonable doubt that null is not a value (because any real value is always equal to itself). And since the RM prescribes that "there must be a value in every cell of a table", any thing that possibly contains nulls, is not a relational thing, and thus the question of 1NF doesn't even arise.
I've heard it argued that Nullable columns in general break the first degree of normalization.
See above for the sound reason underlying that argument.
But in practice it's very practical.
Only if you're immune to the headaches that it usually causes in the entire rest of the world. One such headache (and it's only a minor one, comparatively to other null phenomenons) is the fact that WHERE x=y in SQL actually means WHERE x is not null and y is not null and x=y, but that most programmers simply aren't aware of that fact and just read over it. Sometimes without any harm, other times not.
In fact, nullable columns violate one of the most fundamental database design rules : don't combine distinct information elements in one column. Nulls do exactly that because they combine the boolean value "this field is/is not really present" with the actual value.
