ITALO train service is great, and since then has been a Trenitalia's flourishing of promo, offers and general increasing of cheaper available seats.

So far so good...

From August 26 it's available a no-stop service between Rome - Tiburtina and Milano - Rogoredo. 2h 45'

Rome - Venice is now available in the booking engine, NTV will launch the service on Oct 3.
The route is Rome Ostiense - Rome Tiburtina - Florence - Bologna - Padova - Mestre - Venice.

Italo from Florence to Rome
 

Just got back from a trip to Italy and we took Italo from Florence to Rome. We opted to book coach class since it was such a short trip. Overall I thought the train cars were nice and in great shape. The bathroom was also very nice and clean compared to previous train trips we've taken in Europe. Overall I'd definitely recommend Italo.

Turin is in the network from December 9.

Italo is my favourite carrier of choice, given how practical connections to the Milan metro area are from Garibaldi and Rogoredo stations.

Where should I be looking for train travel this April between Rome, Florence and Venice? We would like to have some flexibility so I don't know how far in advance reservations need to be made. Or is there some sort of pass? We will be in Italy for about 9 days.
Is there a website that lists all the carriers or should we just be focusing on one?

There're 2 carriers:
http://www.italotreno.it/
http://www.trenitalia.com/

High-speed service pricing is quite similar to the airline pricing.

Agreed on leaving Milan, but that advantage is negated if you then have to lug it over to Termini for any regional travel, etc...

Do trains not run every day between major cities? I threw in some random dates for April between Rome & Florence and both came up with no service. And when there was a day with service, there was only one train (I expected several times to choose from) and it was more expensive than I had imagined. Is there any kind of unlimited train travel pass?

How are Termini and Centrale different? Anyway Italo doesn't get to Termini. The NTV bases are Tiburtina and Ostiense.

Sure, but

currently, bookings are available only until March 18:
http://www.italotreno.it/EN/timetabl.../overview.aspx

Bookings up to May 6 will be available from Jan 22.

thank you!
I am still trying to find out if there is some type of 7-10 day tourist pass for unlimited train travel. anyone?

Right, that's what I mean; if you arrive at Tib. and have a Trenitalia regional departure (which I usually do, anyway) it's much more convenient to have arrived at Termini itself...