I throw mine in my in basket at home. Then when I'm processing my in basket I enter them into the Palm Desktop, which enters them into the Palm on HotSync. Then I toss the cards in the wastebasket as part of my ongoing jihad against paper accumulation.
On GTD Fast, DA says he staples them to a letter sized sheet of paper, which gets filed according to the event on which they were collected. If a group of cards were collected at a particular conference, he creates of file for that conference, and all the cards are stapled to the paper, and the paper is dropped into the file.