It is proven technology, I had a motorcycle with a belt-drive back in 85-86 (Kawi GPz-305) and Harley-Davidson/Buell has used nothing but belts on their motorcycles for 10-15 years now. And as much as I appreciate Trek forging ahead with new technology, here are some of the drawbacks I can see:

Drawbacks that apply to both bikes and motorcycles:

1) the belt abhors a small diameter. So there is a limit to how small you can make the small sprocket (front for motos and rear for bikes). So if you want to have the equivalent of 39/16 gearing on a belt-drive system, you might need to go 58/24 or something. And that big front sprocket might look pretty weird. Look at the size of the rear sprocket on a Buell Blast (motorcycle) if you want to see how goofy a too-big sprocket looks.

2) you are limited as to your gearing choices. Of course having horizontal drop-outs gives you a little leeway, but at some point changing your gearing will require a different sized belt, and it is not anywhere as easy as just taking some links out of a chain.

Drawback that only affects bicycles:

3) unlike motorcycles, most bicycles use the chain-drive to change gears. So unless there is some further development in terms of gearing built into hubs or even cranks (planetary gears??) belt drive is limited to single-speed and hub-geared bikes. So even if this works great, it will still be a niche within a niche.

Anyhow, I applaud Trek for going forward with this, and who knows, maybe in 20-30 years chain-driven bikes will be regarded as antiques.