I usually wouldn't mind buying all my train tickets in Italy, but for a few reasons I wanted to buy the ticket for my first train ride (Roma-Lecce) the day I arrive. I didn't want to be stuck without at ticket at the train I wanted, and by buying in advance, you can take advantage of the 20% cheaper Amica fare.
But from reading Slow Talk, I got a little worried - there were many threads were people lament not being able to buy tickets on the TrenItalia website with US credit cards. Two of the latest threads were these: Trenitalia booking tix b4 you go and TrenItalia online tickets. The last thread provides a link to a post on the Fodor travel forum: Trenitalia – How I Successfully Bought Train Tickets on the Web, where a poster by the name of Michael explains step by step how he was able to buy tickets - and it seemed extremely difficult!
So Sunday morning I armed myself with a US credit card, a Norwegian credit card, and determination. I registered, got a password on email (right away), and started the purchasing process. I found the train I wanted, clicked on it, selected my class and fare, entered my card information - and that was it. I was the holder of a second class reserved seat ticket from Rome to Lecce.
I don't know if I was extremely lucky but none of the bad stuff happened - and I did not use a CitiBank card, as recommended, or call my credit card company to tell them that I was going to buy from an Italian site, or any of the other things that were supposed to guarantee success. I did do the process in Italian but I can't imagine that makes a difference. Edited to add: I used the American credit card, a Visa card from a smaller bank.
I plan to buy tickets for another train ride as well, because it is on May 1 (Labor Day) and because I want to get the Amica fare. I am crossing my fingers that it will be as easy!!