I'm trying to wrap my head around this still, but I have read from reputable sources that 720p and 1080i have very similar bandwidth, but that similarity is more or less averaged out among different types of programming. For instance, sports which pan around a lot will most likely make less efficient use of 1080i.
As far as I can tell, only the parts of the image which have actually moved from frame to frame need to be changed when displayed in H.264. 1080i interlacing can easily be observed on a computer that is playing a 1080i video, and is either too loaded down or not up to the task. The moving (transient) components will have a "horizontal blind" appearance, only showing up in the parts of the frame which have actually moved, not the entire image at all times.
So the basic concept is that 1080i does much better than 540p, on the average. Those who say that the average similarity in bandwidth between 1080i and 720p alone tells us that there is nearly a 33% improvement on average over just doubling/halving the lines. But nevertheless it could be closer to that for sports.