I have a 48inch LG Plasma set to 1080i (TV is 4 months old).
I have watched NFL, CFL, NHL, Golf, Nascar on TSN HD, CBS HD, NBC HD, and Global HD. I have never at anytime seen any macroblocking at all. There are times where I might get the swirlly grain in the background that is indicative of MPEG4, but never in any form that would reduce the quality of the main action.
I made a point of watching over the weekend to see if there was anything even on Sports Central (TSN HD) since they have all the sport highlights and and nope not a single issue.