I have a 1080P lcd and watch OTA HDTV on it on a regular basis. Some channels broadcast in 720p, some in 1080i. From couch distance (8 feet or so) the difference is negligable as far as clarity is concerned. If I get right up to the display though, 1080i looks a bit better. I think this is so because a 1080p display does not need to scale a 1080i signal (in my case, I have the digital tuner box set to output 1080i so I think the tuner is doing the scaling but scaling is scaling).
Where 720p is supposed to be better is with fast moving video. Some sports (especially sports with small objects like tennis, golf, hockey, etc) and action movies benefit from 720p because 1080i flickers more when there's more motion on screen. I can't really confirm this though, I don't watch that kind of stuff very much.