Eric Bohelert points out that a pro-Park51 rally appears to have drawn more of an audience than the anti-Park51 rally that took place over the weekend. Even the New York Post, which has played a central role in amplifying the conservative misinformation surrounding the project has higher pro-Park51 numbers.
I don't think this refutes the polling showing most New Yorkers are opposed to the Park51 project, but I suspect the anti-Park51 crowd may have overestimated its salience as an issue. The doldrums of late August provided a perfect vacuum for a manufactured conservative controversy, but most Americans are still occupied with weathering the jobless aftermath of a punishing recession. Also the presence of Quran burners and pseudo-fascists like the English Defense League make it kind of hard to pass off the event as not motivated by anti-Muslim animus. It's easy to see why even someone opposed to the project might not want to show up if it means hanging out with that crowd.