If your budget is only $600, you might try looking for a NOS frame on eBay. I found a 90's era schwinn steel road bike frame and won the bidding at $50. With $20 for shipping, it came out, all told, to a $70 frame; I thought it was a steal, considering my other options were in the $200 range. If you shop selectively, or have parts lying around at home, you can make a pretty good quality bike out of $530.

But $600 is pretty restrictive for the road bike category. Most decent stuff will put you in the $1000-2000 range. You might want to have a look at the Raleigh catalogue. Raleigh is weighing in on the commuting market and has some cheap offerings. I got my Rush Hour fixed gear/single speed bike for $550 back a couple years ago. Besides all the "spiritual" benefits of fixed gear bikes, one practical benefit is that they are cheap! If you don't have a lot of hills in your area, seriously consider the fg/ss option.