As Gavin Andresen pointed out in his initial comment about Cloud Mining some four odd months ago, GAW could also utilize merkle sum trees to provide cryptographic proof of work to once and for all confirm that they are not operating with fractional mining,
That would not require them to disclose a mining address. It would require competent programming on GAW's part.
As Josh mentioned in this very thread, if they are running fractional mining, then Josh should be in jail for fraud.
(I do not pretend to understand what Gavin meant, but then hey, I don't run the blockchain. I do believe that he knows what he is talking about)
Another way they could prove they aren't a fractional reserve/ponzi would be to give customers the ability to point the hashrate at any pool. (verify the hashrate via the pool readings.)
They wouldn't even need to disclose a mining address which mr hashking clearly wants to avoid at all costs.