It's not the "movie" that supports 1080i, it's the player. I believe the PS3 (like other Blu-Ray players) will
interlace the 1080p video read from the disc to 1080i and output that to your TV. If the PS3 can do that well, the picture should look great. If the PS3 doesn't do that well, you can possibly try setting the PS3 to output 720p and let the TV either display that natively or scale the image to its native resolution, which might be or include 1080i.
Believe it or not, the PS3
used to have an issue with outputting 1080i in that it wouldn't when connected to CRT HDTVs that accepted 1080i but not 720p.
Peace...