Just got an alert this morning from Fandango that ticket sales for The Avengers in 3-D has begun in the San Francisco Bay Area... but so far only at the Century Theaterin Daly City!
I checked for the NJ/NYC area, and didn't see any listings of theaters or ticket sales, so try and check your area, let's see where else sales have begun.
Other theaters have not been listed as of yet, and neither have the 35mm and the IMAX 3-D versions been listed for ticket sales. So, make sure to register for an alert, because who knows when that will be coming around the bend!
Also, no runtime info was posted as far as I could tell; feel free to add any more info in the comments, of course.