yep, it's the compression (and maybe the p's & i's)
In my local market (Denver), the NBC affiliate splits its signal to allow for their "NBC Weather Plus" channel. No big deal for most broadcasts, but for football I get lots of pixellation. Switch over to Fox or CBS or ESPN: little to no pixellation.
Fox seems to have the best signal for sports, which I attribute to 720p broadcast signal -- is it accurate to say that 720 has less info (as far as lines of resolution) but displayed more completely (720 "new" lines at a time vs. only 540 with 1080i)? -- I remember reading somewhere that 720p was superior to 1080i in broadcast signals due to the bandwidth limitations with broadcast TV.
But I only have a Sony rear-projection 3-LCD (not even an XBR), but who can beat it when your wife wins it on a grocery store scratch ticket, eh? Hard for me to complain about the occasional pixel box!