Not sure what nights it's on, but during good weather starting about May, there used to be open-air tango on the Quai St. Bernard, on the left bank of the Seine near the University and Jardin des Plantes, one night a week. Other nights have other dances, including West Coast Swing. You might check that out.
Thanks for answering. Now, since 2003 the original poster has probably made his/her own opinion.
Things change, some milongas close (Bahia Blanca), some new ones open. The open-air milonga along the Seine (quai Saint-Bernard) is threatened more and more each year because of the alleged noise. August is when most milongas are closed. This website http://www.tango-argentin.fr/ lists all of them and is updated on a daily basis.