My RDU->FRA paid flights this spring on US had end to ends on Lufthansa via use of UA ff miles FRA->ATH. USAIR was nice enough to even tag our baggage for the complete itinerary RDU->ATH at check in when I showed my other itinerary. I believe it is common for flyers to book end-to-ends during the winter and shoulder season sales from the northern European destinations offered to warmer places that aren't available in the transatlantic winter sales.