Sorry, no progress on this. Although it might look like it, It isn't easy to implement.
Currently only oderators and Admins have the tools to 'merge' duplicate venues into one. Care to help? We could make a thread on the setlist.fm forum (http://www.setlist.fm/forum/setlistfm/website-support) for duplicate venues ... or feel free to post your ideas on how to improve the current situation
for the time being, if you want to, you can send them to help [at] setlist [dot] fm and we'll put it onto the setlist.fm fanpage on facebook
@JustDerek: as leglessmoof said, concerts on the same day for the same venue are in the upper left corner - usually it should be pretty clear who opened for who but if it isn't there's always the possibility to add an @Info[Lamb of God and Mastodon opened] at the very end of the setlist ... and in one point you're wrong: the site's called setlist.fm and not setlist.com ;-)