In the past I'd been doing cardio first or on alternate days...but the past few months, I started doing cardio *after* weights. My reason for doing cardio first? Because all of the GOOD machines were occupied by the time I finished lifting!
Now it's not as much of a problem - mostly because two major gyms opened within a 5-mile radius (a Gold's and a JCC). Since they're brand-spankin' new, they're a bit nicer and are offering 'grand opening' deals...there's definitely been a noticeable drop in the number of folks at my 24 Hour Fitness. Not enough for them to go out of business, but enough where I don't have to wait for a Precor elliptical - at least not very long
So now I generally do 30 or 40 minutes of weights, followed by 25 or 30 minutes of cardio. Except on Thursdays, where I do 50 minutes of cardio and some abs.