Shades of meaning

Today I was asked a tough linguistic question: under what circumstances should proven be preferred to proved. According to the Guardian Style Guide, proven is a creeping feature and should not be used as past form of prove, except in certain cases. My dictionary, on the other hand, claims that both may be used interchangeably. I, however, have a feeling that it is acceptable to use proven when the voice is passive, but that proved should be used when the voice is active. Thus,

The theory was proven by empirical evidence
I proved the theory with empirical evidence

Not that I'm convinced that I'm right. Or that it really matters.