I'm finding these publications extremely useful:
Skip over everything else and go directly to "Seeding and Transplanting" section. According to the Lettuce publication, fall lettuce should be directly seeded July 25 to Aug 10 in Pa. and cooler areas (I'm thinking central to northern Jersey or first average frost in early Oct) and Aug 5 to Aug 20 in warmer areas (south Jersey and Pa. to Del/Md or first average frost in late Oct).
Don't pay any attention to most of the rest (or actually read them -- seed treatment, Weed control, insect control.... Ugh! I preferentialy buy organically grown produce, but after reading these RECOMMENDED agricultural practices, I don't know if I can eat another conventionally grown veggies and fruits at all ever again without heebee jeebies
Also checkout Southern Exposure Seed Exchange's fall/winter guide:
Note the significant difference. I believe it's because Southern Exposure is expecting a more individualized care of your garden than the Rutger's commercial recommendation, and I think they are looking at extended fall harvest past frost until hard frost and freeze, probably with protection using spun bonded covers and low/high tunnels. AND overwintering in some cases.
And Johnny's Select Seeds' Succession Planting Calculator Spreadsheet (I like this one better than the fall planting calculator spreadsheet because you can plug in the days to maturity of your specific varieties and control the fall adjustment factor) to find out the last planting date:
You can also start the lettuce seeds outside in the shade, like Bobberman mentioned elsewhere, or inside under lights -- but it IS starting to cool down to low 70's and high 60's overnight so I tend to think outside might work better... Or do the opposite of early spring >> bring inside in A/C during the heat of the day and put them outside for the night.