It depends on the type of work you're doing. Most jobs in the US fall under the 'Fair Labor Standards Act' and are classified as 'exempt' or 'non-exempt'. 'Exempt' jobs do not require payment for overtime work.
The classification of a job as exempt or non-exempt is not arbitrary but I don't know all the rules governing that. I would wager that the majority of people reading this page who live and work in the US would have an 'exempt' job, though.
All "Computer Professionals" are exempt, so that would cover many of the people on this site. The other exemptions are rather broad and would probably cover most of the rest.
Almost all jobs where you aren't paid hourly (so basically retail, manufacturing and construction) are exempt. I can't think of a single white-collar job that isn't exempt from the overtime requirement.
Not sure if it's Federal or CA law but you have to be making over a certain amount, though, to be exempt. In my last company that way everyone in the engeering dept. making under 70K (so, all of QA and tech support)
For the Federal law, it's $455 per week, so a vast majority of people make enough. I wouldn't be surprised if CA had a higher minimum, since their laws are tend to be more worker friendly than the rest of the US.
The classification of a job as exempt or non-exempt is not arbitrary but I don't know all the rules governing that. I would wager that the majority of people reading this page who live and work in the US would have an 'exempt' job, though.