Does anyone know of a shuttle service from DaVinci airport to Termini??
There is an express train for 11 E which goes there several times per hour.
A slight correction to Bob's post. He is correct about the cost (11 EUR), but not about the timetable. The Leonardo Express train runs every 30 minutes and the trip takes 31 minutes.
We took this train last September. It was very convenient but let me warn you it was hot!! No air conditioning, or if there was it didn't function very well. I don't sweat much but I felt like I had been in a steam room. Still, I'd do it again. Much more fun than a cab. Be sure to follow Rick's instructions about the ticket machines and make sure you validate your ticket.