According to the latest report form the U.S. Bureau of Labor Statistics, In 2002, 5.3 percent of U.S. workers across the nation held more than one job. (The bureau defines multiple jobholders as "employed persons who had either two or more jobs as a wage and salary worker, were self-employed and also held a wage and salary job, or worked as an unpaid family worker and also held a wage and salary job.")
The report didn't break its information down into job-type but I wouldn't be surprised if people in the IT industry weren't even more likely to moonlight in that field. Many folks in the IT field are there because the work excites them and their broad-based computer skills translate well to freelance activities. That's exactly where things can get sticky for the employer.
If you're a programmer at a bank who, in your off-hours, works baking donuts, then there's no problem since there wouldn't be a conflict of interest. In other words, the bakery isn't a direct competitor of the bank. But if you are the web designer for X company, you could be breaking some ethics violations if you help a possible competitor set up their web site on the side. It's an issue of trade secrets being place into the wrong hands.
Employers create moonlighting policies for non-compete reason as well as the fear that someone working two jobs may be too tired and inattentive to do well in their primary job. Also, employees may unethically spend time that should be reserved for the primary job working on issues having to do with the secondary job (taking phone calls from clients, doing research on company computers, etc.)
Many HR pros caution against creating moonlighting policies because it creates an atmosphere of distrust between employees and employers. They recommend using a non-compete contract instead. As an employee, what is your opinion of moonlighting policies? Are you OK with them or have you seen some that are too strict?