As far as conventions go, there have been attempts to hold major gatherings, but no conventions. Check out Diaperfest for some of the larger ones - 50+ have attended their functions.
I've heard other parts of the country where they have had large groups attending parties - i.e. New England, Chicago, and then there are other areas where they have regular monthly munches - diaperedinoregon2, Indiana - "Hoosier High Chair" Toronto Age Play Group, Still in diapers New York...... just to name a few. It doesn't take too much effort to get groups together, if there is an interest. Check out the groups above and you can find out about them, if you like.