Sura 2:23-24
And if ye are in doubt as to what We have revealed from time to time to Our servant, then produce a Sura like thereunto; and call your witnesses or helpers (If there are any) besides Allah, if your (doubts) are true. But if ye cannot- and of a surety ye cannot- then fear the Fire whose fuel is men and stones,- which is prepared for those who reject Faith.
Sura 10:37-39
This Qur'an is not such as can be produced by other than Allah; on the contrary it is a confirmation of (revelations) that went before it, and a fuller explanation of the Book - wherein there is no doubt - from the Lord of the worlds. Or do they say, "He forged it"? say: "Bring then a Sura like unto it, and call (to your aid) anyone you can besides Allah, if it be ye speak the truth!" Nay, they charge with falsehood that whose knowledge they cannot compass, even before the elucidation thereof hath reached them: thus did those before them make charges of falsehood: but see what was the end of those who did wrong!
Sura 11:13-14
Or they may say, "He forged it," Say, "Bring ye then ten suras forged, like unto it, and call (to your aid) whomsoever ye can, other than Allah!- If ye speak the truth! "If then they (your false gods) answer not your (call), know ye that this revelation is sent down (replete) with the knowledge of Allah, and that there is no god but He! will ye even then submit (to Islam)?"
Sura 17:88
Say: "If the whole of mankind and Jinns were to gather together to produce the like of this Qur'an, they could not produce the like thereof, even if they backed up each other with help and support.
Sura 28:48-49
But (now), when the Truth has come to them from Ourselves, they say, "Why are not (Signs) sent to him, like those which were sent to Moses?" Do they not then reject (the Signs) which were formerly sent to Moses? They say: "Two kinds of sorcery, each assisting the other!" And they say: "For us, we reject all (such things)!" Say: "Then bring ye a Book from Allah, which is a better guide than either of them, that I may follow it! (do), if ye are truthful!"
Sura 52:33-38, 44
Or do they say, "He fabricated the (Message)"? Nay, they have no faith! Let them then produce a recital like unto it,- If (it be) they speak the truth! Were they created of nothing, or were they themselves the creators? Or did they create the heavens and the earth? Nay, they have no firm belief. Or are the Treasures of thy Lord with them, or are they the managers (of affairs)? Or have they a ladder, by which they can (climb up to heaven and) listen (to its secrets)? Then let (such a) listener of theirs produce a manifest proof....Were they to see a piece of the sky falling (on them), they would (only) say: "Clouds gathered in heaps!"
It seems clear from the above that the Challenge is to produce anything written by any creature or collaboration of creatures that rivals the Qur'an, not just something written by a human being. The idea as it is usually presented is that the Qur'an is especially or unusually suitable to its purpose, the conveyance of 'irfan (knowledge of God). For instance, it is said to draw with its beauty, to be exceptionally memorable, to have the power to convert, etc. In other words, the argument is that we have reason to think that the Qur'an itself, in Arabic, as a recited book, is theophanic. Muslim commentators have actually done a great deal to specify and clarify the Challenge: the doctrine of nazm (selection and organization of the words), and discussions of such things as conciseness, poetic structure, wit, are obvious cases. The idea is not that any one of these things individually makes for the unique inimitability of the work, but that it is the whole. There is a whole field of study on it.
Now, the argument presented by the Challenge is clearly valid: if there can be a work such that only God could make it exist, and it exists, then God did it. The inference is a causal argument; the idea is that the Qur'an, like the creation itself (cf. the Sura above), is something that could not actually exist unless caused by God. As to the sort of causal argument it is, it is clearly a design inference; what motivates the argument is the beautiful organic functionality of the recited work. This is, I think, a potential weakness of the way the Challenge is usually formulated; design inferences are rather tricky things.
From the Christian perspective, I think one's view would have to be something like this: The proper response to the Challenge is not to quibble about the excellence of the Qur'an, but to introduce people to the real Theophany, the real Perpetual Miracle of Revelation; the argument should be that compared to the Divine Word, the Qur'an, however much may be attributed to it, is but words on the page. And if they deny Him, let them produce a revelation like Him.
