I have the same question. The HD-ready TV will arrive Saturday, probably during the Busch Race. I'll be without an HD tuner. My understanding is that Fox does not broadcast in HDTV, but limits its signal to 480p.
Perhaps some who've watched the NFL on Fox can add some insight.
I expect NBC will have HDTV for the season in July.
Don't forget that TNT also airs a handfull of the late-season races. Hopefully the powers-that-be at NASCAR were watching TNT's superb NBA All-Star weekend!