One major area of employment in Rome is the tour guide industry. There are a lot of guide companies in the city, varying from upmarket organisations that will only hire academics with relevant qualifiactions, to fly-by-night companies who'll hire anybody who's young, good-looking, willing to hassle tourists at the major sites, and able to memorise a script. Particularly at this lower end of the scale, working as a tour guide can have a high burn-out rate.
There are also issues with the tour guide licenses in Rome, and not having one can mean a hefty fine; see below for more on this. That said, there are always companies willing to employ unlicensed guides, especially in the busy season. You'll find vacancies listed - often thinly disguised - on sites such as Wanted in Rome.
Tour guide licenses
Officially, all tour guides in Rome need to have a license, and the police do occasionally pull over unlicensed guides and fine them. (This is even a problem with small, discreet groups, as the police are apparently trained to recognise guides by hand gestures, etc. They also often work on tip-offs from disgruntled licensed guides, who don't like to share their territory.)
Fines are handed out to unlincensed guides every day, but one example is Giovanna Pizzorno, who was fined €172 for apparently 'pointing out the historic and artistic beauty of the Trevi Fountain without authorization.' You can read an article about this here. In 2005, The New York Times carried a story about the crackdown on illegal guides.
The tour guide license itself is difficult to get hold of, requiring you to sit exams which are held very rarely, and always in Italian. The licensing system is apparently illegal under EU law, and have recently changed, in theory to make it easier for foreigners with suitable qualifications to get the license. It's still not easy, though; information on the license is hard to find, and you need to get your university transcripts translated into Italian. Rumour has it that a percentage of the candidates are also failed on the exam despite getting the required score, simply to keep the licensed guide numbers down.
There are a lot of very bad guides in Rome, and a lot of very good ones, and a fair and efficient tour guide licensing system could really help visitors to Rome. At the moment, however, there still seems to be a long way to go.