The Andover office might take on new traders, I know the owner had in the past and there is a Worldco office that might in Boulder. At least it is worth checking out but I don't think the odds are very high in either case unless you are putting up risk money. But then again if you do that I would not consider that true prop anymore. Also if you go that route I think a Bright office is still around there.
Bright has an office in Denver, specifically Lakewood. There are about 15 traders in the office, and it's probably one of the best groups of intraday traders that I know of. Most of the traders are very experienced and trade a variety of styles. There's also a good mix of system trading, automation, discretionary, tech analysis, pairs, opens, scalping, etc. As for the people, every single one of the guys and gals there are a real pleasure to work with, share ideas, and trade beside. I know this because I drive 70 miles, each way, to trade there everyday.....
I've traded in the Denver Bright office serveral time when visiting over holidays and during our dreadful Arizona summers. Dave's right, it's a great office. One of their best, akin to Las Vegas, Manhattan and Omaha! But, you need to be a 'true professional' - Series 7, $25 to $50k and more discipline than I brought to the table a few years ago!
ETG is near the city, as is Carlin, and Andover is as well. If your new to the city or don't know about it there is an extremely active group of traders there called the Denver Trading Group which meets about every two weeks. Check them out if you don't already because they bring some pretty good speakers. I belive one of the Andover guys is speaking at one of their presentations in June. Not sure though.