I've searched the branch for this, but can't find an answer. The current LP China book says the train link from the airport to the city will open in 2007, and also that the underground train Line 5 is under construction. Have either of these been completed and opened yet?

Line 5 is supposedly set to open in September, and the air train in July of 2008 along with lines 4 and 10 (I think it's 10- the one the follows East 3rd Ring then goes north of the 3rd ring), and the Olympic line.

Thanks everyone for the answer, even if it isn't the one I wanted to hear. What's the best way to get from the airport into the city? Bus, I guess, but where do they stop in the city? I'm probably going to be staying near Tian Tan.

Forgot to say, my plane arrives late evening. Does the bus run late, or will I have to pay for a taxi?
Do a google and you will find maps of the airport bus routes (sorry can't remember them).
If it is too late for the bus then sorry but you will have to take the taxi option.

Taxi fare from the airport to the center of Beijing was about 150 RMB, from memory, about a year ago. I have heard of people being charged 500RMB for the pleasure of this trip.

You only pay RMB 500 if you are foolish enough to take one of the illegal cabs. Go to the taxi queue, and make sure he puts the meter on. A taxi to the city is about RMB100, depending where you are going. That's about US$12 - 13. I live on the east side of the city (same side as the airport) and it costs me between 65 - 85RMB for a cab from the airport. Or take the bus, provided your hotel is close enough to one of the stops.