Not a huge story, but I heard this week that United Continental will no longer grant a discount by checking your bags online and paying in advance. The first checked bag used to be $23 and the second bag $32; however, the new rules maintain the $25/$35 fees for the first and second respective checked bags no matter how you pay.
It looks like Delta still maintains a discount for booking online in advance, but I question how long this discount will last now that United Continental has made the switch.
For reference, the following is a list of other airline baggage fees as of the date of this post assuming non-elite status and baggage under 50 lbs.
Airline – 1st, 2nd, 3rd+ Baggage Fees
US Airways – $25, $35, $125
American Airlines – $25, $35, $100
Southwest Airlines – Free, Free, $50