I'm going to "stick my neck out" and address a very controversial subject: whether it's better to take the plane or the train between Rome and Venice. I expect that there will be many responsive postings which will argue in favor of one method of transportation or the other.
Let's start with the train. Beginning tomorrow, December 13, there will be HOURLY AV "Frecciargento" trains from 6:45 to 18:45 (northbound) which will take 3 hours, 48 minutes to reach Venice S. Lucia Station from Roma Termini. The "base" fare in second class will be 73 Euros, discountable 30% to 51 Euros with an advance purchase of at least 15 days. Through February 28, 2010, there will also be a "Maximum 48 Euro" promotional fare with an advance purchase of midnight prior to the date of travel. Both the 30% discounted fare and the 48 Euro fare are CAPACITY CONTROLLED, so they may be unavailable for the train or date selected. The 48 Euro fare is totally non-changeable and non-refundable (and expires after February 28, 2010 unless extended).
There will also be one Intercity (IC) train between Roma Termini and Venezia S. Lucia, with travel time of six hours, 13 minutes, and a "base" fare of only 40.50 Euros (discountable 30% to 29 Euros with an advance purchase of at least 15 days). The lower cost of the IC train comes with a greater travel time compared to the AV train.