From the RCA website (link):
Observance of Israeli Holidays in the Diaspora
Sep 30, 2009 -- In many years, there is confusion about the proper dates for observing Yom Hashoa, Yom Ha'atzmaut, and Yom Yerushalayim is the Diaspora. After discussions and consultation with our Av Beit Din, Rav Gedalia Dov Schwartz, the position of the RCA is that Diaspora Jewry should celebrate and observe these days on the dates the Chief Rabbinate of Israel designates for them. The Chief Rabbinate publishes these dates on its website.
Yom Hashoa should be observed on Monday, Nisan 28/April 12
Yom Ha'atzmaut on Tuesday, 6 Iyar/April 20; and,
Yom Yerushalayim on Wednesday, 28 Iyar/May 12.
For 5771/2011, the corresponding dates are
Monday, Nisan 28/May 2;
Tuesday, 6 Iyar/May 10; and,
Wednesday, 28 Iyar/June 1.