Also to add to what Quantum said, the difference between 1080i and 1080p is practically negligable on PCs.
From my understanding, computers can only truly output progressive material. My CRT TV can only display 720p or 1080i, but if I download a "1080p" movie, it still shows up fine on my TV. The resolution is all that matters.
The only catch is that when my TV is set to 1920x1080, the refresh rate is 30Hz. Since it's interlaced, the TV can only handle half the normal refresh rate. When I set the TV to 720, it runs at full 60Hz. For PC games, that might be a big deal. However for movies, most movies run at 24 FPS anyways, so running at 30Hz is fine.
:/, I'd disagree with resolution being the only thing that matters. Simply put, per frame 1080i only put out 540 lines of information, where as 720p obviously outputs 720 lines, so certainly during fast moving sequences 720p holds an advantage, but for slow moving stuff 1080i does actually output more information per frame so can be higher quality. As for computers outputting, yes they output progressively, but unless you set your program to deinterlace the file (which can some times make it look better depending on the type of show and the deinterlacing algorithm) it will still just output interlaced as a subset of a progressive manner (i.e every line will be updated, but only every other line will be updated with 'new' information).
This is at the moment somewhat a moot point, as almost all 1080p and 720p T.V shows that are encoded are taken from 1080i source, the source will be the best quality but will often be 3 - 4GBs for 45min shows, so upto you if you have the hard drive space. But is fairly important when you downscale or re-encode Blu-Ray or HD DVD sources.
As for refresh rates, the ideal refresh rate is 120 Hz, this fits nicely as 24 x 5, 30 x 4 and 60 x 2 (old PAL stuff be damned). When you're playing a film in 30 or 60Hz there is a certain amount of pulldown, which can lead to a slight jerkiness during scenes. I much prefer the way PAL deals which this, as it is 50Hz, is simply doubles every frame to get 24 to 48fps then increases the speed of the movie and sound by 4% to get to 50fps. Audio quality takes the very slightest loss as everything is pitched 4% higher, but very very few people can tell the difference.