http://www.hometheatermag.com/gearworks/1106gear/
That explains it. 1080i is essentially 1080p that only displays every other line, halfing the speed in which data is fully displayed. Since movies only display at 24fps, this has zero impact on the picture quality. All 1080p does is replicate the same frame multiple times to compensate.
So, TV broadcasts are all 720p/1080i, the 360 and PS3 appear to be stuck around 720p in large-environment games, and 1080i is just as good as 1080p for movies.
It makes it hard to justify the extra money needed for a 1080p set unless you're planning on keeping it long enough to play the next generation of gaming systems, which will be the only real use for 1080p sets in the forseeable future.
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