Mike astutely observes that the lesson of Jamie Lynn Spears may be that the "country is more forgiving of a teen pregnancy than we might suspect." I agree, but I'm not sure it helps Palin and her daughter. There are lots of "sins" for which Americans are far more forgiving than they used to be--homosexual affairs, various addictions, etc.--but which Americans still see as disqualifying, or at least damaging, for a national politician.
I'm not saying Bristol Palin's pregnancy is in the same league as these other examples. Or even that it'll prove damaging. Just that you can't necessarily extrapolate from popular feelings toward celebrities and their family members to popular feelings toward politicians and their families. (People have low expectations for celebrities--that's one reason the McCain camp is trying to make Obama into one.)