If you have a separate seat booked for your baby then you should be able to take the car seat onboard unless airline policy dictates that the particular seat you have is not suitable - I would check this with the airline before flying.
If the baby is sharing your seat then you should check with the airline as to whether they have space to store the seat onboard or whether you need to check it in.
To date I have only taken my car seat on long haul flights and have never had a problem taking a car seat onboard - however I have always checked beforehand with the airline that this was ok
If you are planning to travel on CX, please check out their website: Cathay Pacific - Helping You Travel : Special Assistance > Infants, Children & Minors. We lugged our car seat on board last year to use on the flight, only to find CX had new seats with airbag seatbelts which cannot be used with a car seat. Now we just check in the car seat as it seems that most of the CX flights we go on have the new air bag seatbelts.