I just got this message from USAirways:

Save time and money when you check in online. Beginning July 9, 2009, you'll be able to pay for checked bags when you check in online – $15 for your first checked bag and $25 for your second. For customers who choose to pay for checked bags at the airport, there will be an additional $5 service fee, so save a few when you check in online!

I guess this is how they are making up for lost revenue from returning to free soft drinks onboard.