Sometimes the cheaper seats on popular flights (ie, Fri/Sat/Sun mid-morning departures) are sold early, and you just can't get the best rates if you think you must travel at those times. Flexibility is helpful in getting the best fares. We have often waited until mid to late Dec to book late January flights, unless there's a very specific date/time you need to travel.
If you start watching the fares now, you'll have an idea of the usual cost and be able to recognize when there's a sale. The current rock bottom $100-$110 fares on Delta (and US Air, Southwest, and others) is unusual --- although it also happened last year. The difference this year is that it's for later in the fall, and I just read that Delta was offering these incredible rates into December and January/February, BUT just for a few hours yesterday.
I got $80 vouchers per ticket this week because Delta offered the low fare on my specific flight for just a few hours and I happened to be home to read about it on the internet. Guess I'm trying to say that it might be worthwhile to monitor this site and some specific airline sites, especially Delta, just in case something like that happens again. There might be a very narrow window of opportunity.