I once asked a cab driver how much he made (in SF), and after humming and hawing, he implied that he made close to $90K/year, mostly tax-free.

Take it for what it's worth (sample size == 1), but there are a lot of people who would love to be driving a cab in SF. Cab drivers who rent the cabs pay $300 for an 8-hour shift on weekends (Friday evening - Saturday morning), and still manage to make enough money to make it worth their while.

Presumably they're making more than their next best alternative.