Probably "best" if you separate weights from cardio, but it won't kill you or anything. How long are your cardio sessions? 10 minutes? 30 minutes? 2 hours? Assuming you want to eat pretty soon after your workout, you're just delaying it a bit.
There's the tradeoff: if you do weights + cardio, your postworkout nutrients get ingested later (e.g., after your cardio is finished). Does it matter in the long run? Probably not much, if at all. It may have an impact if you're doing something stupid like running for an hour or two after weights. If you're lifting, then drinking a PWO, then waiting so you don't puke (what? 30 minutes?), and then doing cardio, you're not really doing "weights followed by cardio" anymore, IMO. If you're worried about it, sip on some whey during your workout. And if your weight sessions are very high volume and brutal, and you're going to follow it w/ 30-60 minutes of equally brutal high-intensity cardio, sip on whey + some carbs during your workout.
Basically, if you've eaten a solid meal w/in a few hours before working out, and you're not doing anything stupid for cardio, cardio after weights will not have a huge negative impact. If you're really worried about this though, play it safe and keep 'em separate. But if after working out is the only time you can fit in your cardio, then do it. And stop worrying about minutiae. You'll be fine.