Apparently the Falcons still want an open air stadium. Rich McKay - who I once shared an elevator with - had this to say today:
"A retractable roof stadium has always been one of the options, but our first preference has been to focus on an open-air stadium ... [o]ur discussions have, to date, primarily focused on an open-air stadium. There are no new developments to share, so it's not appropriate for me to comment further than that."
I'd be surprised if McKay isn't in-the-know, thus it appears the team is anti-dome and anti-retractable roof. Retractable roofs are sketchy at best. I live in Milwaukee, and I frequent Miller Park during baseball season. While the beer is plentiful and the brats are delicious, that roof is a maintenance nightmare. From time to time, it just gets stuck. While fitting the Dome with a retractable roof is the cheapest option, the lost revenue during construction and the maintenance costs may prove prohibitive.
Here is the article where he's quoted.