I saw the following article on CNN.com this morning and thought I'd post it for anyone intending to travel soon between Naples and Sorrento on the Circumvesuviana:
CNN.com Circumvesuviana Derailment
I poked around a little on Google News for more information, and one article from an Italian news source indicated there would be a bus service that would transport passengers inbetween the parts of the track that are currently not operable by train.
Just a heads-up for anyone traveling in the area soon - you may want to monitor the news for further developments.