Honestly, I think the Nielsen ratings are off on this show. According to Nielsen, 24 gets about 12 million viewers (using data from last season) and wasn't even ranked in the top 30 shows for the season. 30 million watched last years premiere, I have a hard time seeing more than half that audience not following the show.
FOX has a gem here, and this year they are promoting it well very again (remember last year's commercials for it during the Super Bowl?) and I can see the show getting its highest ratings ever this season. But like I said before, the ratings for last season have to be wrong. I mean 30 million watch CSI each week, and that show isnt nearly as interesting or action-packed as 24.