I hear they only have 8 episodes (representing 8 hours out of the full 24) completed, and considering the heavily sequential nature of this show, they don't want to air the eight episodes and then take the show off the air for who knows how long.
This WGA strike is really starting to impact things in a bad way. I was really looking forward to season seven of 24. The trailer looked really good and it seemed like it might make up for the weak previous season.
Interestingly, neither the official site nor the trailer site have been updated to reflect the fact that the show will not be airing on it's scheduled premiere date of January 13, 2008. I'm guessing they either just haven't gotten around to it or maybe they're waiting to see if this whole thing is settled in time to allow the show to air after all.
Source: Fox News