Here's an interesting idea, a "here, buy this bike" thread for new cyclists on a budget. often times craigslist yields some pretty good deals to the would be cyclist with limited funds. so post your city and best bike 600 bucks will get you today.
from portland oregon, id totally buy this bike for 300 bucks (perhaps cheaper, seller says make offer)
http://portland.craigslist.org/wsc/bik/3958076874.html