A cite would be good. There is nothing on Bungie's site, nor Wikipedia. The last interview I saw was
Eurogamer's from 30th April where the level cap was still not revealed.
A level cap of 20 just looks way too low unless the level of experience needed to progress to each level isn't so much on a curve bit basically doubles each level. That's be crazy brutal though. And the number of skill unlocks, assuming at most one per level, suggests more than 30 levels. There was certainly no more than one skill unlock per level in the alpha.