According to that map, the stations at Hongdae and Gongdeok won't be open until 2012 and the end of 2011, respectively. According to the Hankyoreh,
Travel between Seoul Station and Incheon International Airport will become faster and more convenient, as the Phase 2 section (20.4-kilometer section linking Seoul Station to Gimpo Airport) of the Korail Airport Railroad opens on Dec. 29. This construction follows the Phase 1 section (37.6-kilometer section linking Gimpo and Incheon airports) that opened previously. It has been almost 10 years since the project started in April 2001.
Travelers can also check in luggage at Seoul Station before leaving for the airport, the Ministry of Land, Transport and Maritime Affairs said Wednesday. They will pay 5,300 Won ($5) for regular trains traveling from Seoul Station to Incheon Airport in 53 minutes, and 13,300 Won ($12) for express trains in 43 minutes.
They just switched over the signs at Gimpo Airport station the other day. Previously the two levels of the station had the express train to Incheon Airport at level B4, and the all-stop train at level B-3, but now the latter level has signs reading "To Seoul Station," with both the express and regular trains leaving from level B4 (the north side of the station is the platform for line 9). I read somewhere (I can't find it now [update: Wikipedia says this]) that when the Seoul Station portion is complete, some trains from Incheon Airport will shift tracks and take Line 9's route to Gangnam.
The construction taking place in the Magok-dong fields (which I've written about before) has been pretty much completed for some time now (though up until a month ago part of the tunnel appeared to still be uncovered), and trains have been doing test runs for a few months now as well. It's disappointing that the connection to line 2 at Hongdae won't be finished for another year or so, but I can't complain about a subway option that can get me from Gimpo Airport to Seoul Station in 20 minutes.