West Side Story was announced for a Nederlander-owned theatre. When a show announces it is going to "a Nederlander theatre to be announced," it means there is currently a tenant in that theatre that hasn't announced a closing.
Looking at the tenants in Nederlander houses, Wicked, The Lion King, In the Heights, The Little Mermaid and Grease all seem reasonably sure bets of running in 2009, so I think those theatres are off the market, and 9 to 5 now has the Marquis. They would have been able to announce the Nederlander Theatre if that's where they were going (nor do I think Arthur Laurents would want the show to play there).
That leaves the Neil Simon or the Palace. Hairspray and Legally Blonde have their weekly ups and downs depending upon tourist trade - but Hairspray has recouped and can withstand a few losing weeks in the winter, while Legally Blonde has not and is in a more vulnerable position. Plus, the Neil Simon has 1,400 seats while the Palace has 1,700 and given the scale of West Side with a cast of 37 (not including understudies) and an orchestra of 30... its producers may be seeking more seats to cover the show's likely enormous weekly running cost.
So, for all of those reasons, I suspect West Side Story will be ensconced at the Palace next spring.
Begin at the beginning and go on till you come to the end: then stop.