In the interest of me not having to look up, again and again, the date of when this is going to happen:
After the rebalancing, which takes effect before the market open on May 2, Apple will make up 12% of the Nasdaq-100.
OK, so after the close on Friday April 29, then in effect from Monday May 2, 2011.
http://online.wsj.com/article/SB10001424052748704587004576243231566493842.html (you might need a subscription to access this WSJ page).
There is other information around if the WSJ is inaccessible:
are two sources (Bloomberg and Reuters). I am sure there will be plenty of nutjobs writing about how its all a conspiracy, but will leave others to link to those types of sources 🙂