i did a month by bike in italy in 2005. my thoughts-
-florence is hell
-amalfi is amazing. salerno to sorrento is the most memorable ride, ever.
-the trains are iffy when it comes to bikes, we rode from one city to the next, typically 3-4 day routes, hopped a train to a different region, repeat... just be flexible and don't count on the train actually having a bike car, despite what the schedule says. no, it ain't germany.
-itineraries don't work and cause stress. be like a leaf on the water, and let the current move you along.
-rome is okay on the bike. it is actually an easy town to navigate (hard to miss the landmarks, imagine...), the roads are wide (!) and traffic is slow.
-venice is best on foot. leave the bikes in padova and take the train for the day.
-naples... skip naples unless you have family to visit, or are adept at kung-fu. it's a festering pit of crime, and you are a slow-moving target who is carrying all possessions with you on your bike, like a fish in a barrel. ymmv.
-ravenna is a nice town to bike in and out of. riding from there to padova through commachio was nice, but resources are widely spread out. also lots of ferry crossings and stuff, best to have a guide. lonely planet was 99% correct on the complicated route, the 1% error being on an interchange entering comacchio. gps fixes that.
-hostels are bike friendly, but can be overbooked more often than not. have a plan b, know your options. some days you just can't ride another mile, and have to drop cash on a pension or worse, a hotel. good for the showers, but expensive.
-every regional seat has a cycling map of their region in their information office, and every town big enough to have a train station offers cycling maps of the area. hit the information offices. i came home with five pounds of cycling-specific maps which were very helpful.
-italian... we are lucky to be saturated with latin roots in much of our language. it's not that hard to get by. then again, i tend to be adept with languages from over 5 years of formal study and several years living abroad. italian was not one of them, but gringo spanish and an eye for word origin is enough to decode what isn't available in english. ymmv.
-aglio. don't assume your food will come with it, you have to specify you want garlic!