The funny thing is Wired magazine predicted this soft-drink development a few months ago in their Found: Aritifacts from the Future column.
The main active ingredient in the weight-loss drink is epigallocatechin gallate (EGCG), a green tea extract that, according to Coke, increases metabolism and by extension burns calories. I don't know about weight-loss activities, but EGCG has been shown to have neuroprotective effects in ALS, have anti-cancer effects and be anti-diabetic (possibly by acting as an insulin mimetic) -- among other effects. It makes you wonder at what point these 'natural' compounds will begin being regulated as drugs, especially when they have such wide-ranging and varied effects and are being used as additives in products that will almost certainly be abused.
As an aside, here's an interesting list of weight-loss drugs and their side effects.