Manhattanhenge 2009 occurred over the weekend. They call it "Manhattanhenge" because the sun becomes centered on New York City's grid at this time. The City is about 30º out of line with the sun's setting and so the dates of "Manhattanhenge" do not fall on the dates of the spring and autumn equinoxes.
On Saturday, May 30, you could see half of the sun on Manhattan's grid (by looking down a city street towards the West), and on Sunday May 31st, specifically at 8:17 pm, you could see the sun in full.
I originally planned to view the sun setting by looking down 57th Street from the east side of town, but I ended up over in Queens on a dock in Long Island City (off of Gantry Park on 50th Ave & 2nd Street). Besides the view of the sun being centered between the buildings on 33rd Street (below), there were also great views of the East River, Queensborough Bridge and the famous Pepsi Cola sign.
More info here on NewYorkology.com via the Hayden Planetarium of The Museum of Natural History.