Bear in mind 1080i as a signal used in broadcast can come about one of two ways:
1 - film which was originally 1080p (24 or 25 frames) is interlaced into 1080i (50 or 60 frames). In this case, a GOOD TV can perform what is known as cadence detection, and can reassemble the frames without any loss.
2 - video which was recorded using a 1080i film camera directly as 1080i 50 or 60 frames. In this case each frame is from a slightly different point in time (rather than above where each pair of frames has been created from a single, whole frame). Here you can get deinterlacing artefacts which I won't go into detail here or I will be here forever!!
For me personally, HD sport should have been 720p50. This means the signal will have a TEMPORAL RESOLUTION advantage i.e. there are 1280x720 pixels, times 50 unique frames a second. If you ever get a chance to see 720p50 you will be amazed just how fluid the image is. 1080i50 has a SPATIAL RESOLUTION advantage i.e. there are more pixels a frame, but half the amount of frames a second. This better where detail is more important over movement.
(HD-DVD and Blu-Ray are 1080p24 disc, the output is either interlaced to 1080i or downscaled to 720p. 1080p50/60 isn't something we can get on disc or over the airwaves)