Good points all around but it is important to note that while people may gravitate towards a field with a supply shortage, they might not make it. This is because there are barriers to entry that prevent most people from obtaining employment in a given field, regardless of their ambition or intent. These barriers could be created by employers/society to maintain quality standards or from the group itself to maintain exclusivity.
That leads into another point, I feel that there's generally three factors that determine how much a job tends to pay.
1. Difficulty - How
hard the job is. Do you really need to be smart to do it, do you need to be strong, fast, whatever? Sports are difficult, because you have to beat out everybody else. Acting is a great example, because it's difficult to be known as the best, who command premium pay, compared to the thousands of lesser actors who usually make next to nothing.
2. Training - The more education/training it requires, the more pay. After all, passing courses, attaining a degree is difficult in and of itself.
3. Hazardous/Nasty - Garbagemen, nuclear power plant workers make more in order to attract people to those positions. Perception is sometimes more important than reality in this.
This is a general career field guide. Factory workers are very productive on average, but it also tends to be relativly unskilled work, thus their pay will tend to be less than somebody like a repairman, who's work is more complicated, but less productive overall. There's always going to be problems getting enough doctors, for example, because the sheer amount of difficult training required makes them scarce, thus they make lots of money, making more people at least consider the field. It's a major reason why getting your bachelor's degree.
Professional athletes, actors, CEO's are explained by an industry obsessed with getting the 'best'. While there are many people in these fields, the category of 'elite' or 'best' ones is much, much smaller. If you survey a large number of football fans and ask them who they think the best quarterback is, odds are you'll end up with only a half dozen names. Having Arnold, Chuck Norris, Steven Segal, Sean O'Conner in a film virutally guarentees millions at the box office, allowing them to command extreme wages.
