I am turning part of my answer on question What rights do passengers have when flights in the USA are delayed?, into a new question.
The EU made very elaborate passengers rights on EU-based airlines. These rights apply as soon as you fly with such an airline, but do the rules still apply if you booked with an EU-based airlines, but the flight is operated under code-sharing with a (non-EU) partner airline?
The rules will apply to the operating carrier. ie, the one that is actually flying the flight - not the one that you booked with.
EU Compensation applies to EU airlines regardless of where the flight is to/from, AND to non-EU airlines for flights DEPARTING an EU member state. eg, a Lufthansa flight between the US and Frankfurt would be covered in either direction, whilst a United flight would only be covered from Germany to the US and NOT in the other direction.
So if you book a Lufthansa codeshare that was actually operated by United, then the regulations would apply only on the legs that depart from the EU, and not for any other legs as United is the operating carrier.
