It's an Italian tradition adopted by much of Europe. It's not an old one however. It was completed fabricated in a book , and then a movie called "I Want You" by Federico Moccia in 2006. There's a NYT article about it: http://www.nytimes.com/2007/08/05/world/europe/05iht-rome.4.6991537.html

That being said, it will surely exist forever because it's an awesome tradtition...