Been meaning to ask this questions for a while...
Last year (August 2010) I flew LIS-EWR-TPA. LIS-EWR was on TP and EWR-TPA was on CO. Both flights were booked under one ticket, however, the EWR-TPA flight was booked as a UA codeshare, operated by CO.
While checking-in at LIS, our bags were tagged to TPA as expected. However, the bag tag listed the UA codeshare flight number instead of the actual CO one. I tried to explain to the agent that the flight was operated by Continental, but she didn't seem to understand.
The bags arrived in EWR, and I cleared customs with them. At the CO baggage drop, I showed the guy our tags and asked again if there would be a problem. He said "Wow, good thing you noticed that. Go over to the desk and have them re-tag your bags".
Sorry if this is confusing, but what would've happened had I not gotten them re-tagged in EWR? Would they have made it to TPA? Seems like a fairly easy mistake to make as a check-in agent.