Nice job, site looks good. I hope it works out for you!
How do you plan to show the transparency from bitcoin to usd to charity? I see that you are planning on doing it, but I think that implementation will be the most important to get a lot of people on board.
We will have PDFs of receipts of transactions in MtGox, and people can see the coins moving from the bitcoin-charity wallet to one of MtGox's. The exchange rate is public knowledge, so the exact amount in USD converted is given. There will then be receipts of donations to each individual charity, which can be verified by simply contacting the charity in question if anyone wishes to do so. You're right that there's a certain level of trust because everyone is not likely to verify every single receipt of USD going to charities - but the option is there. In all likelyhood, the conversions to USD and donations to charities will only occur a couple times a year. Because of that, the amount of verification needed is minimal for any member of the public.
So in summary, the trust is there because everything is publicly verifiable.