buy train ticket now or when we get there?

If you want the cheapest ticket, you need to buy now. The mini fare is available up until 24 hours before but it is capacity limited so it might not be available if you wait till then.

The cheapest ticket is the Reginale R train ticket. No seat booking, no reservation. You buy the ticket. It will last for two months. You can ride any R train. If you buy the IC or ES, you will automatic make the seat booking and reservation. But you only have one hour and thirty minutes to rebook if you miss the train. Some time it is very hard to rebook within one and half hour. It is always a long line in the booking service counter. The R train is a lot cheaper than the IC or ES. Maybe half the price of the ES. For IC or ES, you maybe able to buy the ticket at the ticket machine for discount ( Promo) price the night before you travel If you buy the ticket before mid-night. Also if the promo is available.
Have a good trip!

I've never been able to book online from the U.S. Has this changed? I don't like booking ahead of time since this takes some of the flexibility out of my itinerary.

Scott, many folks are now reporting that you can book on the Trenitalia Website with your US credit cards! The website says that only Italian AMEX cards are accepted but some people have reported that yes, their US AMEX cards worked! So it seems Trenitalia has solved their credit card issues (well, to some extent).

Lori, How many days will you be in Rome prior to travel to Venice? You could easily buy the ticket when you arrive in Rome, preferably a few days before travel. You can purchase either from the staffed ticket office (often long queue's) or from one of the automated Kiosks. That way you may be able to get the discounted "Mini" fares. Have a look at This Website for lots of great information on travel in Italy.

Thanks for the info everyone, I bought the tickets today, they now accept credit cards on the website, and I am glad I bought now, they went up 10 eu in 2 days! so if anyone else is trying to make this decision, buy now!

Thanks for asking the question, Lori, and thanks for all the replies. I had the same question. I've also been attempting to buy tickets online (from the US) on the Trentitalia website, but it keeps "timing out" and giving me an error message after I enter the 1st passenger name (we have 4 total). Perhaps I should just keep trying! It's good to hear that it does seem to work! And, worst case, we can buy tickets when we get to Italy. :) Happy travels!