RE: Null values

I agree we should merge the definitions. As a basis, expressing "nullness"
as a function of a 403 or 404 response is good. Then we can identify (as
5.5.3 does) situations that could lead to this.

Jim, I'm missing your point about 5.5.3 not being DAV-like. The phrase
"property/properties defined on" is used all over the DAV draft. 

UNKNOWN may be a misnomer. When I added that I meant that server is unable
to retrieve the value and this is a catch-all for a variety of conditions. 

-Saveen