If you want to get to/from Sorrento/Rome without having to transfer trains in Naples or ride the local train down to Sorrento (with theft risks), or pay 100 E for a private driver from Naples, the Marozzi Bus service direct from Rome train station to Sorrento may be for you. 3-4 hour ride. Problem: website only in Italian. Here are two posts that tell you how to use the website. I have also read that you may be able to wait to buy tickets until you are in Italy, at a travel agent or the bus kiosk/station.
Personally I haven't used this yet but just thought I'd post this.
One person on tripadv complained that AC wasn't really up to it during summer; may be a better option outside of summer.