UPDATE 7/12/12 6 p.m.: At the corner of Six and a Half and 56th street (the avenue is a pedestrian walkway) a crowd of perhaps two hundred gathered. Some were following the Reddit thread in real-time, and the original poster, delverofsecrets, said he would be arriving, even though he was nearly an hour late.
Delverofsecrets turned out to be Ali, who declined to give his last name (although some enterprising Redditors could probably dig it up). He showed the note, proving it was in fact him, and repeated the story of the man handing him the note and the $50 bill on the subway. Asked about the meeting today, he said he didn't know about that either. "It might be a friend of mine trying to mess with me," he said.
Ali added that he didn't expect that many people to show up. "I thought maybe 10 people might be here." As to why he arrived closer to 5 p.m. than four, "I'm just chronically late."
Nobody ever solved the mystery of "Bue Jay." A man dressed in blue, who was raggedy-looking, did appear near the corner at just after 4 p.m. After being briefly mobbed, though, it seemed he wasn't the right person, "Mr. Input."