There are Forex clearing-houses all over the world. How can we have a Forex clearing-house offering Bitcoin as tradeable currency pairs?
Well, the Bitcoin exchanges are a form of a forex clearing house. At the moment, however, Bitcoin is not really ready to be traded alongside all world's currencies at some more major exchanges - the big players could sway its value up and down at a whim with a fraction of what money they have. Before that happens Bitcoin still needs to grow and become more noticeable. However, there isn't much stopping someone from incorporating Bitcoin into a new forex.
Essentially, it is a tradeable currency pair today, just that there is no leverage (or very little leverage) compared to what you see with forex.
Bitcoin is different enough from foreign currencies that it may take a very long time for Bitcoin to be considered "investment grade".
In the meantime, a plethora of alternatives can flourish where stability can exist from the vast number of ways it is being used. For instance, ICBit.se's futures contracts based on the BTC/USD, and now their Gold/BTC and Crude(WTI)/BTC futures contracts as well, are innovations that put bitcoin to use in new ways which allow to some degree a method of shorting bitcoin with leverage (in the case of the Gold/BTC futures contract) or going long with leverage on the BTC/USD (in the case of the BTC/USD futures contract).
MPEX offers CALL and PUT options on the BTC/USD with front-month and next-month expirations. They also offer bond tranches where the proceeds are used for settlement of the options business. These are just some of the first innovations (excluding some false starts, like Bitcoinica and the investment funds on the now-defunct GLBSE).
Existing forex outfits will probably see the opportunity when some bitcoin-specific exchange market begins to grow rapidly, but there's nothing much of the sort just yet.