One more question for TraderThom ...

So if arbitrage trading involves keeping large sums at a number of exchanges, in order to be able to quickly capitalise on opportunities ...
Is that "our" (investors') coins on the exchanges? And therefore at risk from a number of companies "doing a mtGox"?
Do we as investors take that risk? It seems that it can't be avoided really, seeing as the whole point of us investing is so that BT have greater leverage to capitalise on arbitrage opportunities (that wouldn't be the case if our invested coins were in cold storage, obviously).

Regardless, I've invested a small sum with BT - I'm willing to take the risk. Once I've recovered it, I'll start investing more, and ride it as far as it goes!