From thefreedictionary.com:
Usage Note: Prove has two past
participles: proved and proven. Proved
is the older form. Proven is a
variant. The Middle English spellings
of prove included preven, a form that
died out in England but survived in
Scotland, and the past participle
proven, a form that probably rose by
analogy with verbs like weave, woven
and cleave, cloven. Proven was
originally used in Scottish legal
contexts, such as The jury ruled that
the charges were not proven. In the
20th century, proven has made inroads
into the territory once dominated by
proved, so that now the two forms
compete on equal footing as
participles. However, when used as an
adjective before a noun, proven is now
the more common word: a proven talent.