LH has a ruling where you are required to show a credit card entered during booking (not necessarily the one you used to pay) as identification during checkin.
However, a recent court ruling (last week I believe) awarded a passenger damages for beeing rejected for not having the Credit Card during checkin available (although that was I believe on Iberia).
So, if they say its mandatory, you might be ok even without it.

(Ruling: sorry, link to a german newspaper: http://www.faz.net/s/RubBEFA4EA6A594...~Scontent.html
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