Hallo,
 
I quite agree with Michael Kirschner's suggestion that noun+that-clause
is an appositional structure.
 
However, I think that the 'relative clause' in
   "I arrived quite late, which annoyed my hostess"
should be interpreted differently. The reason is that the antecedent is
not always a sentence. In fact, it is typically a whole paragraph. And
when it is a sentence, this sentence is just a very short paragraph
(similar to very short sentences like e.g. 'Stop').
 
On type of paragraph has the structure TDAC or
    topic
    description
    antithesis
    conclusion
 
Which-sentences of the type discussed fill the conclusion-slot in this
paragraph structure. As only topic- and conclusion-slot are obligatory
paragraph-slots, while the other two are free (similar to adverbial of
manner slots in sentences), we often find paragraphs that have only two
slots, and in the case of
   "I arrived quite late, which annoyed my hostess"
 
they have the appearance of (complex?) sentences, although they are
something quite  different.
 
That much for my 'maiden' speech on this list.
Almost anyway, here is a
 
P.S. on Max Morenberg's suggestion that the relationship between the
following sentences is the one the sentences illustrate.
     It surprised me that he ate the pizza.
     The fact that he ate the pizza surprised me.
Talking about apposition (and me not being a 'generative grammarian') -
why don't we just call this an instance of apposition?
 
    It/that he ate the pizza  surprised me.
 
The subject slot is filled by an appositional structure, which, though,
is split up by the second appositive going to the end of the
sentence.
 
  Burkhard Leuschner
  Paedagogische Hochschule,  Schwaebisch Gmuend,  Germany
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