I would say Leah has been learning how to sit up by herself for a few weeks now. Tonight she spent about an hour just sitting on the floor playing by herself. She only needed help after falling over 2 times. I even walked out of the room for a few minutes (with pillows all around to pad her fall of course) - and she did great! She even took a bath tonight in her "big girl" tub for the first time since she has been sitting so well.
Any my baby continues to grow up. Milestone after milestone.