I decided I was actually gonna sleep train this one, do the Ferber thing, put her down, let her cry, comfort without picking up, let her cry again, etc (and doing it at the right age, instead of waiting til the kid is 10 or 12 months and just letting them cry it out). So last night, I nursed her and then picked her up and put her in the playpen. She cried (but didn't scream) for about two or three minutes after I left the room, and then was quiet. When I went into check on her 10 minutes later, she was upset, so I rubbed her back and sang her a song, and then walked out of the room. She stopped crying as soon as I shut the door, and when I checked on her 15 minutes later she was asleep.
Tonight, I did the same: nursed her, sang her a song and then put her in the playpen. She cried until I shut the door on my way out. I never heard another peep out of her, and when I checked on her 15 minutes later, she was sound asleep.
She's so very, very happy, always full of smiles. She loves to squeal at her siblings. The above picture is what we call her "happy screech": she gets in this mode where she just starts emitting this high-pitched squealy shriek to make sure everybody knows how glad she is to be alive. If we could just convince her that a bottle of formula is not an insult to either her taste or intelligence, she'd be just about perfect.