Aerobic exercise should come before weight lifting, not after. (This is, of course, debatable. crazcarl

) You've already put a strain on your body's glucose supply (glycogen stores) by lifting those weights, which, unless you can get enough carbohydrates and protein in the right window of time, will be catabolic.
The formula is:
Muscle Protein Balance = Protein Synthesis - Protein Breakdown
If you don't get enough of the aforementioned macronutrients, your body will be forced to break down both fat and muscle to supply itself with energy. However, it is difficult to gauge how much of these nutrients (especially carbohydrates) you will need during your time at the gym. If you get the amount right or slightly more, you can do weight training and then your cardio without a problem.
So the compromise we can make is if your nutrition is good enough, you can do both. It is, of course, easier and less draining to do your cardio after a weight lifting session.
Personally, I don't usually do cardio on weight-lifting days. If I do, I make sure to keep my carbs up before and after the session.
Bottom line is, cardio after weight-training only if you're constantly supplied with enough carbs. Otherwise you're risking a negative muscle protein balance, which means muscle loss.