We ended up purchasing our tickets 3 months in advance through RailEurope.com. We returned home from our trip a few days ago, so I thought that I would report back on the Cisalpino trip.
Well, the trip was absolutely beautiful. However, despite the fact that we were told to expect both a dining car and food service in the cabins, we got neither. The dining car was locked for the entire 9 hour ride (yet there were passengers in there), and the food service cart only came through our (First Class) cabin. The reason the cart didn't come by more frequently is that they had run out of food! During the 9 hour ride, we were only able to scrounge up 2 sandwiches for our family of 5.
I asked someone who worked on the train, and they told me that they no longer use the dining car, except for "special excursions" (e.g., the people I saw seated there), and that he had no idea why there was so little food for the trip.
All in all, it was still a great trip. But, we were disappointed that we hadn't come on board with food. But, given how the dining car was touted, we had decided to splurge on a nice meal on the train.