Versions of ffmpeg cvs after 5/30/05 seem to have problems with the way duration and bitrate are calculated. I have submitted a bug report to the ffmpeg-devel mailing list. I'll post more info here as it becomes available.
Ok, a change was made to ffmpeg last night that fixes this problem for .mov, 3gp and mp4 movies. I've updated my bug report with a problem avi movie so hopefully they can get that fixed and I can go ahead and release version 0.4.4.0 or ffmpeg-php