this was a nice little discusson at the VF boards this week as well. i think most of us agreed that we do weights first b/c we are strong and it burns the glycogen stores so when you get to cardio you go into fat burning mode. i don't know if that statement is 100% but i do know that trying to lift after doing cardio is very very hard for me so i find it best, for me personally, to do weights first after a light cardio warm up then go into cardio afterwards. usually my cardio after weights will be 20-30 mins but intense. but its all up to the person, so you might want to try to play around with it and tell us what you think works for you.
I joined the gym recently and got several pieces of information about exercise and one thing I got says this:
"Doing your weight training program before you perform your treadmill session, stationary bike, or any other aerobic work out is smart. It will enhance fat loss, improve strength, and even reduce over-training which can cause muscle loss..."
I like a modified "weigths first" routine, because I want to focus more on strength. My modification is to do a rather long cardio warm-up, like the first 12 minutes of Cardio Kicks, because I personally work better when I'm a bit warmer (and with age, it takes a bit longer to get there!).