Zurr ibn Hubaysh said: "I heard Ubayy ibn Ka'b saying - and it was said to him that 'Abd-Allaah ibn Mas'ood said:' Whoever follows the Sunnah will 'catch' Laylat al-Qadr! '- Ubayy said: 'May Allah have mercy on him, he did not want people to take it for granted and only stay up to pray on one night. By the One besides Whom there is no other god, it is in Ramadaan - he was swearing without a doubt - and by Allah, I do know which night it is. It is the night in which the Messenger of Allah (PBUH) commanded us to pray (qiyaam). It is the night the morning of which is the twenty seventh , and the sign of it is that the sun rises on that morning white and without rays. '"
(Hadith - Muslim and others)
However, the timing of the night Laylat al-Qadr has not been narrated with certainty, and its timing may not be the same every year. The wisdom in this is that the Muslims will be more inclined to devote themselves in worship during the whole last 10 days, rather just on one night, as would be the case if its timing were known for certain.
http://www.youtube.com/watch?v=_YRV_LiHIZU