From the New Oxford American Dictionary:
For complex historical reasons, prove developed two past participles: proved and proven. Both are correct and can be used more or less interchangeably: this hasn't been proved yet; this hasn't been proven yet. Proven is the more common form when used as an adjective before the noun it modifies: a proven talent (not a proved talent). Otherwise, the choice between proved and proven is not a matter of correctness, but usually of sound and rhythm—and often, consequently, a matter of familiarity, as in the legal idiom innocent until proven guilty.