They're going to have a reserve fund of BTC / USD to make exchanging easy, and I'd assume that will have a rate that most merchants use.
Will $150million USD be a big enough reserve? Assuming anyone can exchange against that, I can imagine that being wiped out in just a few hours of a bitcoin whale deciding they want to cash out...
They can control the spread on exchanging against their reserve
We have not received much information, but from what I understand, the accounting currency would continue to be the USD.