Well, assuming Bitpay can afford to meet the USD payments. Behind the scenes they can't actually convert Bitcoin to USD instantly, so they're basically eating any change in the Bitcoin-USD rate between the transaction happening and them cashing out. If there's a big enough crash or they receive so many payments that the market cannot absorb the Bitcoin sales, they could easily find themselves unable to meet their obligations. (Things actually looked a bit hairy when BFL used them for payments just because of the sheer amount of Bitcoin they had to convert in a short period.)