> By that time, who knows what the BTC exchange rate will be.
This isn't relevant if you do the following:
1) Transfer USD to Bitstamp from your bank account
2) Buy BTC with your USD on Bitstamp
3) Transfer BTC to Mt. Gox
4) Sell BTC for either EUR or USD on Mt. Gox
5) Transfer EUR/USD to your bank account
In this scenario you are only exposed to exchange rate risk from the time it takes you to transfer BTC from Bitstamp to Mt. Gox and sell it (I think Mt. Gox requires 6 confirmations, so that's - on average - less than an hour currently).
As I see it the largest risk is Mt. Gox defaulting on its USD or EUR obligations, in which case you'll lose everything.