This is so true.
We returned from HK to SFO a few days ago on SQ 2 in J, and learned this the hard way. Wanting to have a relaxing time to lounge at the SQ Silver Kris lounge, we arranged for a Rolls Royce transfer from the HK IC to HKG. We arrived at the airport at 4:45pm, and even though the SQ counter was open, they could not check us in until 6:45pm
They said that it is due to the security procedure post 9/11, and if we did not have any luggage, they could actually check us in right then and there. We had about 10 pieces of luggage with us, and luckily the porters that met our car was still with us, and offered to store the luggage and then meet us at the check-in counter at 6:45pm when it reopens for the USA bounded flight. The check-in agent gave us HK$70 vouchers per person, and told us that we should be back in line for the check-in when it re-opens.
We didn't realize she meant the
LINE 
! After burning more than 2 hours in the limited pre-security shops and restaurants, we returned to the check-in counter only to find a long queue with passengers waiting for the check-in counter to reopen. There are no separate queue for premium class passengers, and we waited in line for more than 20 minutes before finally getting through luggage screening and checked in. Having return to the US from many European and Asian destinations, this experience was by far the most inefficient I have every encountered. While SQ on board service is very nice, I am very very hesitant to take SQ again from HK.
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International Airline seatmaps
Originally Posted by hugochan
This site will be useful regarding Airport Express:
Cathay Pacific and Singapore Airlines will not allow in-town check-in for US bound flight.