Zona Traffico Limitato- what is this???

joan, The links provided by Ron and Kent should answer your questions. The ZTL areas are especially prevalent in Florence, but they exist in other towns as well. They are not intended to scam tourists, but rather to limit cars in the historic parts of each city. Italian drivers (and others from Europe that frequently drive in Italy) are somewhat familiar with them so can more easily avoid them, and local residents can obtain exemption permits. As someone else mentioned, the Cameras have no way of distinguishing between tourists or others, so everyone gets "nicked" equally! When you obtain the International Driver's Permit which is compulsory for driving in Italy, you'll be provided with a small chart that shows some of the road signs in each country. The links will show more specific information on ZTL areas. Cheers!

This post has pictures, maps, and info on the ZTL's in Rome and should answer most of your questions.

Here's a prior discussion thread that provides information about ZTL's: http://www.ricksteves.com/graffiti/helpline/index.cfm?fuseaction=readtopic&topicID=28422&page=2

"Are they clearly marked, or is it a way to get $$$$ from unsuspecting tourists?" I'd say a bit of both. They are clearly marked, but as a tourist in an unfamiliar area, dealing with Italian drivers, motorini, pedestrians, navigation, and trying to read a sign in a language you probably don't know very well... let's just say your odds of successfully avoiding one after seeing the sign (if you see the sign) aren't encouraging.

.....on the other hand, if you know what an international 'do not enter' sign looks like, you've got it made. Everyone knows what signs look like before they start driving around, right???

Yes the signs are easy to miss, no it's not a tourist rip-off (although tourists are often caught with ZTL fines). Here is some advice in trying to figure out ZTLs: 1. ZTLs are always implemented in heavy traffic or difficult traffic zones: 1a - historic town centers, with narrow streets are good bets.
1b - big cities prone to congestion are another good bet. 2. If you see cars circulating freely, it is NOT an indication or clue that you're not headed into a ZTL. Lots of cars have special permits to allow passage into ZTLs... It's best to one-by-one check out what the ZTLs are for the places that you will be visiting.

The automatic cameras don't distinguish between tourists and non-tourists. These cities would be overwhelmed with gridlock traffic in their centers if they didn't have some laws restricting it. The positive side is that it makes these centers much better for tourists who are on foot.

I would say to expect a ZTL in every place you're likely to visit. I've been in a few very small towns that don't get the tourist crowds that do not have ZTL's. You're not likely to be visiting one of those. In my experience the ZTL's are not always clearly marked. Sometimes the signs are posted high on buildings, signs are twisted away from oncoming traffic or falling over, bushes/trees blocking view etc etc. It can sometimes be pretty hectic trying to look out for those while looking for street signs and avoiding pedestrians/scooters/bikes. My general rule of thumb is if I get into an area that looks old (yeah I know...that's two thirds of Italy) I start trying to make an extra effort to look out for 'em or start looking for a place to park and walk to where I wanna go.

I've read many posts on Trip Advisor about people receiving tickets for hundred of Euros months after returning home from Italy. I have been listening to podcasts from several years ago in preparation for my upcoming trip to Italy an it is not even mentioned when they speak of driving in Italy so I assume it has become more prevalent in the past few years as well as a good way to booster their economy. I assume the locals know where the zones are and carefully avoid them but for confused travelers they may be hard to avoid. Does anyone know if the zones are marked on recent GPS systems? That would be a good system of avoidance that I would certainly use if I were driving in Italy.

You can download ZTL info here: http://www.gps-data-team.com/poi/italy/safety/ZTL-IT.html