I work full-time (significantly more than 40hrs/wk), and I train hard. I run sub-14 5000m, and am hoping to go much faster. I did spend about 14 months after grad school (when I was 25) running full-time, living with my parents, doing a little tutoring for cash. My running didn't improve the way I'd hoped it would, but it was definitely a great year in many ways.
Some points (most of which have been made above):
a) Depends on the job. Mine is very flexible. I work big hours, but I spread it over seven days. So on Tues and Thurs I come in late after my morning runs, then leave for 3 hrs in the afternoon to go to workout. But I'm in for four or five hours on Saturday and Sunday afternoons.
b) I work and run, but I do NOTHING else. It's very difficult sometimes. But it's hard enough serving two gods, three is out of the question.
c) Prioritize. Right now (racing season) I wouldn't miss a workout for anything. Even if it meant looking bad at work. In September, on the other hand, I'll bust my hump a little more at work, because base training will be a little more low-key in terms of specific workouts being vital.
d) THough I work full-time, I have to say that the point "Joe Shmoe works full-time and is fast, therefore everyone who doesn't is a pussy" is stupid. Of course if you're running full-time (or close to it) you can focus more on extra recovery, stretching, weights, and so on. Of course, you probably can't afford massage. Which bring up...
e) There's a lot of pressure when you're running full-time, and you spend your last nickel to get out to Stanford or wherever. With a job, I feel more relaxed about running, which helps me race better. If I fly somewhere and race poorly, at least the money is no biggie.
f) Oh yeah -- never compromise on sleep. "Your body learns to do with less sleep etc." is bullshit. With a job, you're probably not getting an afternoon nap, so a good night's sleep is vital. You need a bedtime!
Overall, I'd say it depends on where you're at. If you're 21-22, have decent potential, and (most importantly), want to give it a shot, I'd say go for it. You've got plenty of years to work full-time later. I'm 27 now, which makes me say to myself "be realistic" when I think about taking a year off to prepare for the trials. But if I were 22 again...