Check out writer Linda Fulkerson's excellent primer on how to break in writing greeting cards. She attended the Erma Bombeck 2006 workshop and picked up detailed insider tips including the best type of cards to pitch and how to present your card ideas.
If you want to get into the greeting card business, this is the info you need.
P.S. - This had a broken link before. I have fixed it, and it should work now.