I'm going next month and am using Vacation in Paris: http://www.vacationinparis.com/index.html

They have a wide selection, are highly recommended on several internet travel forums, and are U.S. based so you handle all arrangements in U.S. dollars before leaving.

So far, everything has gone very smoothly - I'll report back on my stay and the apartment when I return in November.

Have you selected a neighborhood you prefer, at least?

I would not rent an apartment in Paris without a/c in July!