Today was the grand opening of the train link between downtown and the airport. For 100 baht (~$3), you can ride non-stop from the city to the airport. Next year the fare will rise to 150 baht (~$5). Soon you will not only be able get to the airport from downtown, but you will also be able to check your bags before you board the train.
While this is pretty cool, it really doesn't have much of an impact on me. There is no sense for me to go downtown before going to the airport. When they finally get the BTS station opened on Chaeng Wattana, then maybe it will help, but even then, its probably a lot easier to just take a taxi.