Just so you know, the only HDTVs that truly display 1080i in native mode are CRT-based. All the LCD, DLP, and plasma-based HDTVs that accept 480p, 720p, and 1080i input (but NOT 1080p) have a native mode of 720p to maybe 768p. That means that when these TVs receive 1080i input, they downconvert to 720p (or in some plasma displays, 768p).
That said, Blu-ray fed to a 720p native mode display looks VERY good. I have such a beast, a 2005 55" LCD-based RP Hitachi with 720p native mode. Ever since I bought an HD DVD player in early 2007 (and now have a PS3 to play Blu-ray), it looks so much better than HDTV or std-def DVD that I *will not* buy any more std-def DVDs and I added HD DVD and Blu-ray to my Netflix profile. I don't watch a std-def DVD anymore unless 1) It's available only in that format and 2) I REALLY REALLY want to watch that movie. Otherwise, Blu-ray fed to my 720p display has spoiled me for anything less.
The interesting thing I've found is that although my TV is definitely run by a 720p-based LCD, when I set the output of the Blu-ray or HD DVD to output 1080i via HDMI, it looks just a *little* sharper than when I output in 720p, the TV's native mode. So don't be afraid to experiment a bit, but *don't* try to feed it 1080p; it may freak out your TV.