This being a Toronto blog, I thought I would give some thought as to what answer I'd give if the same question was posed for Toronto.
You can give your answer in the comments, but you ought to do so before reading further. My answer is below and I've found that seeing someone else's really makes it hard to come to your own conclusion.
I agree with the notion that I read in Yglesias' comments: your answer depends on how well you know the city.
For example, I imagine that someone who doesn't know Toronto well would meet at the base of the CN Tower. It's an obvious landmark for an out-of-towner, just like I might meet someone at the Washington Monument.
But this answer only came to me when I put myself in the shoes of an outsider. Otherwise, I would have never thought of the CN Tower, just like I rarely think of it in any context anymore.
I did come up with a real answer -- and before you read it you ought to go put yours in the comments -- but I'm not sure whether it really makes sense as a general response, or if it is just something particular to me and my friends because of where we live and the kinds of things we do. But if I had to guess a meeting place it would be the corner of Bloor and Yonge. That's where I usually meet my friends already -- the northeast corner, to be exact.
Another good answer would be the corner of Queen and John. But something tells me that having more than one answer defeats the purpose!
As for the time of day, I wasn't able to think about it. Yglesias spoiled that part for me before I even considered it. He writes: "...this originates with Thomas Schelling who asked people the New York City version of the question. The most popular answer there was Grand Central Station at noon, which is certainly the obvious time for any city."