No, there is no Blizzard source yet, but the change was
just made with one of the latest beta builds. Why would they bother to lower the experience needed to level so late in the beta if they had no intention of making it live? And when have they not nerfed prior expansion experience when a new one was released?
The announcement on lowering XP needed for Wrath came out after 4.0.1 was already on live servers and about 3 weeks before Cata (4.0.3a) actually went live -
http://wow.joystiq.com/2010/11/18/wr...reduced-by-20/