Around the 13th century, although not related to Marco Polo! No one's quite sure where it came from.
Now the more interesting thing is that until the 19th century, pasta was not cookied in the fashion we do now, they cooked it for HOURS until it was "fondente" or "melting." An English visitor in the 18th century compared it to porridge. In the 19th century cooking books start to refer to cooking pasta until slightly soft, although the particular phrase "al dente" didn't appear until after WWI.
Pasta also wasn't customarily sauced, it might be eaten with a variety of toppings, a small amount of cheese was common. Poor Italians also didn't eat pasta with forks as they were, well, too poor to have forks I guess. If you know Pulcinella, symbol of Naples, many pictures of him show him eating long cooked pasta with his hands. That was a way to eat it! Although Italians who had forks did eat pasta with forks very early, at least by the 16th century. With the boom in heavily sauced pasta over plain pasta in the 19th century (including the introduction of the now almost ubiquitous tomato sauce) and a gradual increase in the quality of life for the poor, the option of eating with your hands was lost, so that by the time the rest of the world started eating Italian pasta it was all forks all the time. Ah well.
edit: forgot my sauces... :)
These two books are from a nice food history series. And I swear I read instructions for twirling long pasta on your fork is included in the 16th century manners book Il Galateo by Giovanni della Casa but I can't find it in there. :(