14 CFR 399.87 Baggage allowances and fees is the applicable US law. It is applicable to passengers whose ultimate ticketed origin or destination is a U.S. point and to both U.S. and foreign carriers. It basically states the baggage allowance and fee rules of the marketing carrier (i.e., the carrier whose code is on the ticket) on the first leg of an itinerary shall apply throughout the entire journey.
It's effective date is 24 Jan 2012. However, the US DOT has decided not enforce section 399.87 until 24 July 2012 as long as the following conditions are met:
First, carriers will be expected at a minimum to disclose on e-ticket confirmations that additional airline fees for baggage may apply and provide a link from the e-ticket confirmation to the code-share or interline partner websites where the passenger can follow a link and find the baggage allowance and fee information. (This is a requirement of 14 CFR 399.85 - Notice of baggage fees and other fees)
Second, carriers will be expected to reimburse upon request any passenger that was not charged the same baggage fees throughout his/her itinerary if that resulted in the passenger being overcharged.
The following site provides access to the actual law, a Powerpoint presentation summarizing the law, and a FAQ document
http://airconsumer.ost.dot.gov/rules/rules.htm