Biblical scholars and theologians generally do not read Pilate's words at John 18.38 as a literal question. Pilate's question, "What is truth?" is meant to be narrative irony used by the author as a teaching device.
Pontius Pilate was not interested in Jesus giving him an answer. Pilate is obviously bothered by the whole ordeal of Jesus' arrest. The various Gospel references repeatedly agree that Pilate sees through the intentions of those who are bringing Jesus to trial, and later Pilate even has to argue with these same people in an attempt to release Jesus, seeing no reason for his arrest.
His expression, which in Pilate's native language would have been "Quid est veritas?" was obviously said in frustration upon realizing that Jesus is just some holy man and not a political threat to Rome as his accusers had stated that he was. Pilate doesn't wait around to receive an answer from Jesus, you will notice, but goes out to announce his decision that his investigation is over.
Yet as mentioned before, Biblical exegetes see this as narrative irony. John's gospel is very sentimental by comparison with the other accounts, and more inclined to literary expression as a result. The author is not merely stating that Pilate asked a rhetorical question and stormed off, but that Pilate is missing the fact that "truth" is standing before him (just a few lines earlier Jesus had stated that he was the personification of truth at John 14.6).
The theology of Christianity is one that believes that sacred secrets are hidden from the eyes of average people who see things that are only on the surface. It teaches that the Jews are blinded from understanding hidden patterns and codified expressions found in their own Scriptures. The author is probably reinforcing this idea with this incident because the "incarnation" of truth is actually standing before him and Pilate does not take advantage of this by asking further questions. The author wants the reader to learn from this lesson of Pilate's lost opportunity and not let the "truth" escape them as did Pilate.
While I personally agree with this interpretation of the author's intent, as a Jew I don't subscribe to the New Testament claims regarding revelation. Instead of hidden and secret revelations only given to a few or hidden from the average person's sight, Judaism acknowledge only community revelations such as performed in public by Moses and the prophets. I also find the belief that the Jews don't understand their own texts and that some hidden meanings are found within like some sort of code that requires Christianity to decode it as illogical.
Regadless of my personal convictions, however, I see the narrative device theory as a better likelihood regarding Pilate's question than the idea that it is some literal question that begs to be answered (and left without one in the Scriptural text that follows).