It's a gamble. Right now there is a fare war going on, in part because US Air is struggling and Delta is having a few problems. So if US has further problems, they may decide to drop fares in desperation, which will likely cause other airlines to do the same.
On the other hand, fuel costs are going higher, and that drives up the fares sometimes. Keep in mind that Delta is struggling for profits too, so they may instigate a fare war at a later time.
Basically, you have to decide whether the price you are getting for september travel is low enough right now, or whether you will chance waiting until later and risk the fare going up.
They say fuel prices will continue to climb until August. There are signs that travel is growing somewhat, too, so I doubt that the fares are going to get much lower than where they are now, so I might not gamble on lower prices becoming available later on. You didn't say where you were flying from, but from BOS to MCO (Orlando) it is running about $130 and normally it runs in the $200 range.