Well if you've ever seen a 1080P display, the difference is stunning. A much more drastic difference than what we've seen in these new HD channels. Some have posted here that 1080P is comparable to Blueray and HD DVD.
It is all a matter of bandwidth. Broadcast TV is limited to something like 19.3 Mbps of data. I've seen Blu Ray going up to 38 Mpbs when sending out a 1080i/60 signal. That 1080i signal when upconverted by my SXRD to 1080p will provide more detail/resolution than any signal over broadcast TV.
They could broadcast 1080p/24 with the 19.3 bandwidth; but it won't provide more resolution/detail than 1080i and all equipment from the studios to your STB would have to be swapped out (i.e., billions of bucks of current infrastructure).