I am new here also and have seen many cards from other designers with quotes on them. All have credit given to the poet. I currently have a card with a quote from James Dean waiting to be approved. We'll see what happens there. If I did not answer your question correctly, there are many, many other designers here ready and waiting to help you. Best of luck!

Hi Mary-Here is a Gov website that may be helpful http://www.copyright.gov/help/faq/
Good luck, cat

Hi Mary,  
 
In general, copyright is still in effect until 70 years after an artist’s death, this varies from country to country and weather the copyright was in effect before or after a country updated its copyright laws. So the older the verse is the safer the ground you are on.  
 
If the artist is still alive or recently died then you could also be stepping on to the wrong side of trade mark law, this is where you are making money off someone else’s fame and could be depriving them of income. A recent case is of an American sport team claming “what’s that” to be their own and issuing legal notice to independent t-shirt manufactures to stop using the verse.  
 
Better safe than sorry and just because some have got away with it up till now will not mean that everyone will.