Perhaps I'm a bit naive, but when I was interviewing with prop firms in Chicago, the best require(d) you to have a very strong educational background (either that or an innate ability to do math and be very familiar with finance). Top tier prop firms here (DRW, Wolverine, etc.) won't even look at you if you're doing poorly in school. This is not to say you can't get a job there, but I imagine it would take some rigorous networking.
College gives you more security and a good backup. If you want to work at a prop firm where you need to put up your own money, or are responsible for your losses while you're learning, then sure - no degree necessary. But if you want to work at a firm that will pay you salary (around 60k starting now), teach you what you need to be profitable - go to school.
BTW, a lot of companies are hiring out of computer science and technically strong majors now-a-days. As someone said earlier, the quant position is very nice.
For the record, the only trading firms that I've been exposed to are those that pay salary, so I'm not sure the success otherwise; but at the top firms, the success rate is very high.