We were in Italy from 18th Oct to the 19th Dec 2002. We crossed into Italy from France by the Col de Agnel, cycled down the spine of the Apennines to a village called Pozilli about 140km east of Rome. Then we got the train to Bari from where we sailed to Igoumenitsa in Greece.
On the whole we didn't like Italy nearly as much as France. Our main problem was that there was simply too much traffic. We kept to the "high ground" which was strenuous but fairly traffic free, although it did involve many passes of 800m or more. Whenever we descended into a heavily populated valley or plain, even the most minor roads were too busy for our taste. The traffic also contained a high proportion of lorries, moved faster than traffic in France or the UK, and tended to pass very close (probably because there was always traffic coming the other way too!).
The people though were friendly, the language isn't too hard if you already speak French or Spanish, and of course there is fabulous art and history to see in places like Florence, Pisa and Rome. We got the train into Rome, I really wouldn't recommend trying to cycle there or to Naples.
As for the practicalities, most significant towns have very good bike shops, although they are geared to the racing cyclist rather than the tourer. From bookshops you can get maps at all scales although they are not as accurate and up-to-date as French or British maps. In the hills you often have to carry a couple of days food because although many villages have shops they shut for lunch from 1pm until 4pm. Fountains are common in mountain villages so finding water is rarely a problem.
Most local trains will take your bike, most express trains won't, but you can sometimes 'hop' between cities via several local trains. Don't expect the train timetable information staff to be able to figure this out, just use www.trenitalia.com and plenty of patience. It also clearly states in the station timetables which trains take bikes. We found the trains to be punctual, staff helpful, and the trains (in contrast to the roads) were completely empty.