http://www.wada-ama.org/docs/web/standards_harmonization/code/list_standard_2004.pdf
Found that through olympic.org, so I'm guessing that's the guidelines they use. While it doesn't specificly state caffeine, but since caffeine is a stimulant it's covered under pharmacologically similar substances. And the list is hardly exhaustive, as the narcotics section makes no ban on the most powerful narcotics existing. There's probably a more detailed list somewhere, or they changed it this year, because the limit was 12ug/ml.