So what was it? Plagiarism? Unlikely, as Gibson's line is quite famous and neither Gaiman or his editors would be that stupid. Homage? It seemed a little out of context. Well, when searching for the exact line online (I am not near my bookshelf), I inadvertently discovered it was a joke.
...
"The sky above the port was the color of television, tuned to a dead channel." - Gibson, Neuromancer
"The sky was the perfect untroubled blue of a television screen, tuned to a dead channel." - Gaiman, Neverwhere
I uh, laughed my head off at Gaiman's line, and I hadn't yet read
Neuromancer then. It might be a time thing; I seem to remember a time when every sf/f geek I knew was quoting the Neuromancer line.
I don't understand why you would feel the need to directly "borrow" anything without proper attribution.
There are a few reasons one might want to. For example, the Gaiman joke -- it wouldn't have worked if he'd explained it. And in Fforde's case, as girlyswot mentioned, the characters wander into books and meet the characters, and overt attribution would spoil the story.
A couple other examples come to my mind. Madeline Robins' wonderful and unavailable

rant
A Point of Honor starts off with a twist on the first line of
Pride and Prejudice. It couldn't be attributed. The reference is entirely clear, but it's a joke that sets the tone of the book, and citing would kill it right there.
Susanna Clarke has an Austen reference in an earlyish chapter of
Jonathon Strange and Mr. Norrell. The narrator overtly mentions that "another author once said", but does not name her, because that would be breaking character.
And in Pamela Dean's
Secret Country books, certain characters are speaking in lines from fiction, (mostly Shakespeare and other classics) mostly out of context and sometimes changed somewhat, and while other characters occasionally note this, they don't stop every single time and say "And that, fair reader, was from...". There is a most excellent plot-related reason for it, and I think she does it successfully.
So I think it depends on how separate from the book the quote is. If it's in a place where you can grin at the reader and acknowledge that this is just a book, then it's possible to say where the quotation came from. If it needs to be seamless, you need a more indirect way to acknowledge a quote.