I imagine they would keep very little btc in reserve, and convert everything to USD (or whatever) as fast as they can receive it?
BitcoinStore's margins are extremely thin and its suppliers (presumably) take dollars (or fiat), so that is quite likely that they convert all bitcoin revenues into fiat upon receive.
Another business whose supply also requires fiat payments but has a 20% net margin might keep a fraction of their bitcoin revenues as bitcoins and convert the rest. Others might hold 100% of their bitcoin revenues since they were hoping to acquire bitcoins for investment purposes and simply not cashing bitcoin revenues out gives the lowest cost method to acquire bitcoins.
There are solutions. An organization that wishes to keep their coins but protect against the possibility of a "non-trivial" drop in the exchange rate can buy PUT options as a hedge. Or the value of future BTC revenues (e.g., if a set number of bitcoins are being earned for rent payments) can then be "locked in" at a certain rate by selling BTC/USD futures contracts on ICBIT.se. You simply are paying up front to lessen the risks of unknowns down the road.
More solutions in this area are coming as well (e.g., CoinSetter.com ).
