My wife and I went to an IMAX showing last night (we had already seen a standard showing on premiere night Thursday 4/25). The auditorium was pretty full for a Tuesday night, and I was impressed with how lively the crowd was for a movie that has been out past its 3rd weekend now. Not obnoxious/rude lively, but all the gasps/cheering that are part of the movie going experience you would expect from an opening weekend.
The film is past 2.5 billion already in 18 days. I think that even for a severely front loaded film, it still has a good chance to pass Avatar. I hope it happens, because this was a great film (slow build in the first hour, but then really picks up and doesn't let up from there). I've seen Avatar and didn't get drawn into the hype around it whatsoever.