Just for grins, I decided to test this... six weeks out from today is 3/14.
RT from my home airport (LEX) to MCO with a return 1 week later:
Depart 2/22 (3 weeks)
Airtran: $197
Delta: $219
United: $286
Continental: $396
Depart 3/14 (6 weeks)
Airtran: $199
Delta: $219
United: $265
Continental: $265
Depart 3/21 (7 weeks)
Airtran: $197
Delta: $253
United: $254
Continental: $310
Depart 4/11 (10 weeks)
Airtran: $207
Delta: $2219
United: $253
Continental: $278
Depart 5/16 (15 weeks)
Airtran: $197
Delta: $219
United: $261
Continental: $261
So the prices at 15 weeks out are slightly better than 6 weeks out. The prices at 3, 7, and 10 weeks are better for some and worse for others.
All prices are using Travelocity (because they'll give me a summary of multiple airlines). You might get better prices by going direct to the airline.
The bottom line is there is NO magic time. Our last flight (June of 2011) I booked ~320 days out. Prices NEVER got as low as they did when I booked.
As has been mentioned, keep an eye on prices, book when you're comfortable, and know it's a gamble. You can book today and prices drop like a rock tomorrow. You can NOT book today and prices skyrocket tomorrow (and don't come down). If you're going to be upset if prices go down after you book, don't look again (unless you're booking SW or JetBlue(?) where you can get credit).