Not sure if this will be a useful data point to anybody since the free checked bag starts in a couple days, but I had a flight on Friday that I was hoping to squeeze a free checked bag out of since it was 4-5 days before they started it.

Long story short, a US Air phone rep told me to ask at the ticket counter. The ticket lady had to ask her supervisor, who gave a rude "um it starts the 30th" answer. While she was gone I continued my work on the kiosk, selecting to add 1 bag. The ticket lady came back, was surprised to see that I added a bag and it wasn't asking for any additional money and just told me to not worry about it and I ended up getting a free bag. Not sure if it was just a fluke or maybe they added the feature early?

Was a nice surprise, though.