if you're a merchant, you will most likely have to liquidate your income, buy whatever merch you're slinging and sell that for BTC.
i've held leveraged shorts to protect my BTC after I received larger orders,
I do not think it's the payment processors' obligation to hedge.
All I could ask is for them to ffwd a % of my incomings to mtgox, thill, bitcoinica (40/30/5) while keeping the rest as balance, thus keeping pressure of a SINGLE market when selling.
afaik you can open a sell-order @ 1 on mtgox so everything you send there will be sold automatically. If I could do this every X minutes, splitting up the 100s of BTC I sometimes receive over time, that'd be grand.
Bit-Pay already offers instant conversion to USD, so merchants get the exact amount of USD they sold for (minus fees). Volatility is a solved problem, and it's important to make merchants aware that they can accept Bitcoin payments without ever owning a single bitcoin.
also, what he said. only that their fees are prett-tty steep (3%) and I care not for your so-called "US-Dollars" - in fact, I can edge out an extra 5% by handling currency conversion myself.